message: ftp: always use lock_guard
diff: @@ -717,11 +717,12 @@ void FtpImpl::_command_timeout()
  {
  if (_last_command_retries >= _max_last_command_retries) {
  LogErr() << "Response timeout " << _curr_op;
  - _timer_mutex.lock();
  + {
  + std::lock_guard<std::mutex> lock(_timer_mutex);
  _last_command_timer_running = false;
  _session_result = ServerResult::ERR_TIMEOUT;
  _session_valid = false;
  - _timer_mutex.unlock();
  + }
  _process_nak(ServerResult::ERR_TIMEOUT);
  } else {
  _last_command_retries++;
  