message: core: fix (un)signed warning
diff: @@ -160,7 +160,7 @@ void SerialConnection::receive(SerialConnection *parent)
  if (recv_len < -1) {
  Debug() << "read failure: " << GET_ERROR(errno);
  }
  - if (recv_len > sizeof(buffer) || recv_len == 0) {
  + if (recv_len > (int)sizeof(buffer) || recv_len == 0) {
  continue;
  }
  parent->_mavlink_receiver->set_new_datagram(buffer, recv_len);
  