Fix UBSan bot by not passing NULL into memcpy src.

[krasin]

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.

[Eugene.Zelenko]

COFF/Writer.cpp

96

Please use !Config->PDBPath.empty(). See relevant Clang-tidy check.

[krasin]

Fix a style guide violation.

[krasin]

Thank you, Eugene. Done.

[krasin]

I have submitted the CL, as it's aimed to fix a bot. Feel free to send post-submit comments, and I will create a follow-up CL, if needed.

COFF/Writer.cpp

void writeTo(uint8_t *B) const override {
     // get a reproducible build
     if (getRandomBytes(R->PDB70.Signature, sizeof(R->PDB70.Signature)))
       fatal("entropy source failure");
     // TODO(compnerd) track the Age
     R->PDB70.Age = 1;
 
     // variable sized field (PDB Path)
     auto *P = reinterpret_cast<char *>(B + OutputSectionOff + sizeof(*R));
+    if (Config->PDBPath.size() > 0)
       memcpy(P, Config->PDBPath.data(), Config->PDBPath.size());
     P[Config->PDBPath.size()] = '\0';
   }
 };
 
 // The writer writes a SymbolTable result to a file.
 class Writer {
 public:
   Writer(SymbolTable *T) : Symtab(T) {}