GCS_MAVLink: correct out-of-space-to-send count

libraries/GCS_MAVLink/GCS_MAVLink.cpp

@@ -120,6 +120,7 @@ void comm_send_lock(mavlink_channel_t chan_m, uint16_t size)
     chan_locks[chan].take_blocking();
     if (mavlink_comm_port[chan]->txspace() < size) {
         chan_discard[chan] = true;
+        gcs_out_of_space_to_send_count(chan_m);
     }
 }