[AST] ASTReader::ReadSLocEntry(): move computation of FirstDecl into the branch where it's used

The existing code is not defined, you are not allowed
to produce non-null pointer from null pointer (F->FileSortedDecls here).
That being said, i'm not really confident this is fix-enough, but we'll see.

FAIL: Clang :: Modules/no-module-map.cpp (6879 of 16079)
******************** TEST 'Clang :: Modules/no-module-map.cpp' FAILED ********************
Script:
--
: 'RUN: at line 1';   /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/bin/clang -cc1 -internal-isystem /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/lib/clang/10.0.0/include -nostdsysteminc -fmodules-ts -fmodule-name=ab -x c++-header /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/test/Modules/Inputs/no-module-map/a.h /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/test/Modules/Inputs/no-module-map/b.h -emit-header-module -o /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/tools/clang/test/Modules/Output/no-module-map.cpp.tmp.pcm
: 'RUN: at line 2';   /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/bin/clang -cc1 -internal-isystem /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/lib/clang/10.0.0/include -nostdsysteminc -fmodules-ts -fmodule-file=/b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/tools/clang/test/Modules/Output/no-module-map.cpp.tmp.pcm /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/test/Modules/no-module-map.cpp -I/b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/test/Modules/Inputs/no-module-map -verify
: 'RUN: at line 3';   /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/bin/clang -cc1 -internal-isystem /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/lib/clang/10.0.0/include -nostdsysteminc -fmodules-ts -fmodule-file=/b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/tools/clang/test/Modules/Output/no-module-map.cpp.tmp.pcm /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/test/Modules/no-module-map.cpp -I/b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/test/Modules/Inputs/no-module-map -verify -DA
: 'RUN: at line 4';   /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/bin/clang -cc1 -internal-isystem /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/lib/clang/10.0.0/include -nostdsysteminc -fmodules-ts -fmodule-file=/b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/tools/clang/test/Modules/Output/no-module-map.cpp.tmp.pcm /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/test/Modules/no-module-map.cpp -I/b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/test/Modules/Inputs/no-module-map -verify -DB
: 'RUN: at line 5';   /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/bin/clang -cc1 -internal-isystem /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/lib/clang/10.0.0/include -nostdsysteminc -fmodules-ts -fmodule-file=/b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/tools/clang/test/Modules/Output/no-module-map.cpp.tmp.pcm /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/test/Modules/no-module-map.cpp -I/b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/test/Modules/Inputs/no-module-map -verify -DA -DB
: 'RUN: at line 7';   /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/bin/clang -cc1 -internal-isystem /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/lib/clang/10.0.0/include -nostdsysteminc -E /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/tools/clang/test/Modules/Output/no-module-map.cpp.tmp.pcm -o - | /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/bin/FileCheck /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/test/Modules/no-module-map.cpp
: 'RUN: at line 8';   /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/bin/clang -cc1 -internal-isystem /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/lib/clang/10.0.0/include -nostdsysteminc -frewrite-imports -E /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/tools/clang/test/Modules/Output/no-module-map.cpp.tmp.pcm -o - | /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/bin/FileCheck /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/test/Modules/no-module-map.cpp
--
Exit Code: 2

Command Output (stderr):
--
/b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/lib/Serialization/ASTReader.cpp:1526:50: runtime error: applying non-zero offset 8 to null pointer
    #0 0x3a9bd0c in clang::ASTReader::ReadSLocEntry(int) /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/lib/Serialization/ASTReader.cpp:1526:50
    #1 0x328b6f8 in clang::SourceManager::loadSLocEntry(unsigned int, bool*) const /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/lib/Basic/SourceManager.cpp:461:28
    #2 0x328b351 in clang::SourceManager::initializeForReplay(clang::SourceManager const&) /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/lib/Basic/SourceManager.cpp:399:11
    #3 0x3996c71 in clang::FrontendAction::BeginSourceFile(clang::CompilerInstance&, clang::FrontendInputFile const&) /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/lib/Frontend/FrontendAction.cpp:581:27
    #4 0x394f341 in clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:956:13
    #5 0x3a8a92b in clang::ExecuteCompilerInvocation(clang::CompilerInstance*) /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/lib/FrontendTool/ExecuteCompilerInvocation.cpp:290:25
    #6 0xaf8d62 in cc1_main(llvm::ArrayRef<char const*>, char const*, void*) /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/tools/driver/cc1_main.cpp:250:15
    #7 0xaf1602 in ExecuteCC1Tool /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/tools/driver/driver.cpp:309:12
    #8 0xaf1602 in main /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/tools/driver/driver.cpp:382:12
    #9 0x7f2c1eecc2e0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202e0)
    #10 0xad57f9 in _start (/b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm_build_ubsan/bin/clang-10+0xad57f9)

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /b/sanitizer-x86_64-linux-bootstrap-ubsan/build/llvm-project/clang/lib/Serialization/ASTReader.cpp:1526:50 in

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@374328 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Serialization/ASTReader.cpp

@@ -1523,9 +1523,9 @@ bool ASTReader::ReadSLocEntry(int ID) {
     if (Record[3])
       FileInfo.setHasLineDirectives();
 
-    const DeclID *FirstDecl = F->FileSortedDecls + Record[6];
     unsigned NumFileDecls = Record[7];
     if (NumFileDecls && ContextObj) {
+      const DeclID *FirstDecl = F->FileSortedDecls + Record[6];
       assert(F->FileSortedDecls && "FILE_SORTED_DECLS not encountered yet ?");
       FileDeclIDs[FID] = FileDeclsInfo(F, llvm::makeArrayRef(FirstDecl,
                                                              NumFileDecls));