fuzzing: Fix protobuf fuzzer errors when using MSAN
Patch libprotobuf-mutator to unpoison buffers obtained from libfuzzer
via LLVMFuzzerMutate. This is required as libfuzzer is usually not
compiled with memory sanitizer support (not even in OSS-Fuzz project,
see https://github.com/google/oss-fuzz/issues/864). Therefore, we
manually mark the buffer as initialized using __msan_unpoison.