Add missing lock

src/modules/mavlink/mavlink_main.cpp

@@ -808,6 +808,8 @@ Mavlink::resend_message(mavlink_message_t *msg)
 		return;
 	}
 
+	pthread_mutex_lock(&_send_mutex);
+
 	int buf_free = get_free_tx_buf();
 
 	_last_write_try_time = hrt_absolute_time();
@@ -819,6 +821,7 @@ Mavlink::resend_message(mavlink_message_t *msg)
 		/* no enough space in buffer to send */
 		count_txerr();
 		count_txerrbytes(packet_len);
+		pthread_mutex_unlock(&_send_mutex);
 		return;
 	}
 
@@ -842,6 +845,8 @@ Mavlink::resend_message(mavlink_message_t *msg)
 		_last_write_success_time = _last_write_try_time;
 		count_txbytes(packet_len);
 	}
+
+	pthread_mutex_unlock(&_send_mutex);
 }
 
 void