Correct test.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@135460 91177308-0d34-0410-b5e6-96231b3b80d8

test/input.output/file.streams/fstreams/fstream.assign/move_assign.pass.cpp

@@ -21,6 +21,7 @@ int main()
 {
 #ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
     char temp[L_tmpnam];
+    tmpnam(temp);
     {
         std::fstream fso(temp, std::ios_base::in | std::ios_base::out
                                                  | std::ios_base::trunc);