Compiler Warning on GOOGLE_CHECK

My compiler noted line 1073 of this file might have meant to use `utf8_check == VERIFY` instead of `utf8_check = VERIFY`.  I think it might be right based on line 1044.

https://github.com/protocolbuffers/protobuf/blob/64dd751868d633421563e53e68da831a0a02c923/src/google/protobuf/compiler/cpp/cpp_message.cc#L1073