mavlink doesn't have already published message

- if the land detector started before the mavlink module it won't have
   a valid message vehicle_land_detected message until published again

src/modules/mavlink/mavlink_orb_subscription.cpp

@@ -39,13 +39,15 @@
  * @author Lorenz Meier <lorenz@px4.io>
  */
 
+#include "mavlink_orb_subscription.h"
+
 #include <unistd.h>
 #include <stdlib.h>
 #include <string.h>
-#include <uORB/uORB.h>
 #include <stdio.h>
 
-#include "mavlink_orb_subscription.h"
+#include <px4_defines.h>
+#include <uORB/uORB.h>
 
 MavlinkOrbSubscription::MavlinkOrbSubscription(const orb_id_t topic, int instance) :
 	next(nullptr),
@@ -195,6 +197,14 @@ MavlinkOrbSubscription::is_published()
 		_published = true;
 	}
 
+	// topic may have been last published before we subscribed
+	uint64_t time_topic = 0;
+	if (!_published && orb_stat(_fd, &time_topic) == PX4_OK) {
+		if (time_topic != 0) {
+			_published = true;
+		}
+	}
+
 	return _published;
 }