Guard the same push_back window in ProtobufSerializer

`readRow` builds a state with `stringToData` and then pushes it into the column
outside any guard, so a throwing `push_back` would strand it. This is the same
shape as the SerializationAggregateFunction sites, but I could not reproduce it:
it needs a protobuf input harness, and the window is one allocation wide. The
change only makes the ownership ordering uniform.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>