Avoid memcpy from nullptr. NFC.

This was caught by the UBSan bot.  When the atom has no size, we would
issue a memcpy with size0 and a nullptr for the source.

Also, this code should never have references inside an empty atom so
add an assert for that while we're here.

llvm-svn: 264115

lld/lib/ReaderWriter/MachO/ArchHandler_x86.cpp

@@ -420,6 +420,11 @@ void ArchHandler_x86::generateAtomContent(const DefinedAtom &atom,
                                           FindAddressForAtom findSectionAddress,
                                           uint64_t imageBaseAddress,
                                           uint8_t *atomContentBuffer) {
+  if (!atom.size()) {
+    assert(atom.begin() == atom.end() &&
+           "Cannot have references without content");
+    return;
+  }
   // Copy raw bytes.
   memcpy(atomContentBuffer, atom.rawContent().data(), atom.size());
   // Apply fix-ups.