[LLD] [COFF] Fix use of uninitialized memory since SVN r375390

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@375400 91177308-0d34-0410-b5e6-96231b3b80d8

COFF/InputFiles.h

@@ -294,7 +294,7 @@ private:
   // symbols in the real symbol table) are filled with null pointers.
   std::vector<Symbol *> symbols;
 
-  DWARFCache *dwarf;
+  DWARFCache *dwarf = nullptr;
 };
 
 // This type represents import library members that contain DLL names