port-serial: ensure the port object is valid after BUFFER_FULL handling

The BUFFER_FULL signal handler is effectively working in the same way as the response buffer processor, in both cases we may have scheduled the completion of the serial command, and that in turn may end up fully disposing the port object. We must make sure the port object is valid for as long as we need it in this function, so we take a reference while processing the response buffer.

Fixes #535 (closed)