drivers: Prevent duplicate reporting of GPS heading

src/drivers/gps/gps.cpp

@@ -882,6 +882,11 @@ GPS::publish()
 	if (_instance == Instance::Main || _is_gps_main_advertised) {
 		orb_publish_auto(ORB_ID(vehicle_gps_position), &_report_gps_pos_pub, &_report_gps_pos, &_gps_orb_instance,
 				 ORB_PRIO_DEFAULT);
+		if (_mode == GPS_DRIVER_MODE_ASHTECH) {
+			// Heading/yaw data can be updated at a lower rate than the other navigation data.
+			// The uORB message definition requires this data to be set to a NAN if no new valid data is available.
+			_report_gps_pos.heading = NAN;
+		}
 		_is_gps_main_advertised = true;
 	}
 }