[lld] Fix -Wstring-conversion in ObjC.cpp (NFC)

llvm-project/lld/MachO/ObjC.cpp:617:12:
error: implicit conversion turns string literal into bool: 'const char[55]' to 'bool' [-Werror,-Wstring-conversion]
  617 |     assert("Tried to read pointer list beyond protocol section end");
      |     ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~