Fix UBSan bot by not passing NULL into memcpy src.

Summary:
UBSan complains like the following:
tools/lld/COFF/Writer.cpp:97:15: runtime error: null pointer passed as argument 2, which is declared to never be null

The reason is that the vector could be empty.

Reviewers: rsmith

Subscribers: Eugene.Zelenko, kcc

Differential Revision: https://reviews.llvm.org/D24050

llvm-svn: 280259