[MachO] Fix UB in memcpy

UBSan complains about a memcpy with a null pointer, so just skip the
memcpy call if the data is empty.

lld/MachO/InputSection.cpp

@@ -26,6 +26,7 @@ uint64_t InputSection::getFileOffset() const {
 }
 
 void InputSection::writeTo(uint8_t *buf) {
+  if (!data.empty())
     memcpy(buf, data.data(), data.size());
 
   for (Reloc &r : relocs) {