Tracker: auto move servos only with valid vehicle position

AntennaTracker/control_auto.pde

@@ -10,6 +10,11 @@
  */
 static void update_auto(void)
 {
+    // exit immediately if we do not have a valid vehicle position
+    if (!vehicle.location_valid) {
+        return;
+    }
+
     float yaw = wrap_180_cd((nav_status.bearing+g.yaw_trim)*100) * 0.01f;
     float pitch = constrain_float(nav_status.pitch+g.pitch_trim, -90, 90);
     update_pitch_servo(pitch);