mavlink shell: check if there's enough free buffer to send the mavlink message

if there is not, the process on the other end of the pipe will just block.
This improves reliability over slow links.

src/modules/mavlink/mavlink_main.cpp

@@ -2225,13 +2225,15 @@ Mavlink::task_main(int argc, char *argv[])
 
 		/* check for shell output */
 		if (_mavlink_shell && _mavlink_shell->available() > 0) {
-			mavlink_serial_control_t msg;
-			msg.baudrate = 0;
-			msg.flags = SERIAL_CONTROL_FLAG_REPLY;
-			msg.timeout = 0;
-			msg.device = SERIAL_CONTROL_DEV_SHELL;
-			msg.count = _mavlink_shell->read(msg.data, sizeof(msg.data));
-			mavlink_msg_serial_control_send_struct(get_channel(), &msg);
+			if (get_free_tx_buf() >= MAVLINK_MSG_ID_SERIAL_CONTROL_LEN + MAVLINK_NUM_NON_PAYLOAD_BYTES) {
+				mavlink_serial_control_t msg;
+				msg.baudrate = 0;
+				msg.flags = SERIAL_CONTROL_FLAG_REPLY;
+				msg.timeout = 0;
+				msg.device = SERIAL_CONTROL_DEV_SHELL;
+				msg.count = _mavlink_shell->read(msg.data, sizeof(msg.data));
+				mavlink_msg_serial_control_send_struct(get_channel(), &msg);
+			}
 		}
 
 		/* check for ulog streaming messages */