AntennaTracker: avoid integer underflow in mavlink txspace check

Tools/AntennaTracker/GCS_Mavlink.pde

@@ -7,7 +7,7 @@
 static bool in_mavlink_delay;
 
 // 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!!
@@ -166,7 +166,7 @@ static void NOINLINE send_simstate(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);
     switch (id) {
     case MSG_HEARTBEAT:
         CHECK_PAYLOAD_SIZE(HEARTBEAT);