Rover: avoid integer underflow in mavlink txspace check

APMrover2/GCS_Mavlink.pde

@@ -10,7 +10,7 @@ static bool in_mavlink_delay;
 static bool	gcs_out_of_time;
 
 // check if a message will fit in the payload space available
-#define CHECK_PAYLOAD_SIZE(id) if (payload_space < MAVLINK_MSG_ID_## id ##_LEN) return false
+#define CHECK_PAYLOAD_SIZE(id) if (txspace < MAVLINK_NUM_NON_PAYLOAD_BYTES+MAVLINK_MSG_ID_## id ##_LEN) return false
 
 /*
  *  !!NOTE!!
@@ -377,7 +377,7 @@ static bool telemetry_delayed(mavlink_channel_t chan)
 // try to send a message, return false if it won't fit in the serial tx buffer
 bool GCS_MAVLINK::try_send_message(enum ap_message id)
 {
-    int16_t payload_space = comm_get_txspace(chan) - MAVLINK_NUM_NON_PAYLOAD_BYTES;
+    uint16_t txspace = comm_get_txspace(chan);
 
     if (telemetry_delayed(chan)) {
         return false;