MavlinkOrbSubscription::update: improve performance & fix corner case

- reorders operations, such that the most expensive one (orb_copy) is done
  only when really needed.
- corner case: when the topic was not advertised yet, orb_stat() would fail
  and then update() was called, which succeeds for the first advertisement.
  In that case the timestamp was incorrectly set to 0 and true was
  returned.
  The next call would again return true, because the timestamp was updated,
  but the topic data was still the same.

Reduces CPU load by ~2% on a Pixracer.

src/modules/mavlink/mavlink_orb_subscription.cpp

@@ -80,22 +80,22 @@ MavlinkOrbSubscription::update(uint64_t *time, void *data)
 	// TODO this is NOT atomic operation, we can get data newer than time
 	// if topic was published between orb_stat and orb_copy calls.
 
+	if (!is_published()) {
+		return false;
+	}
+
 	uint64_t time_topic;
 
 	if (orb_stat(_fd, &time_topic)) {
 		/* error getting last topic publication time */
 		time_topic = 0;
 	}
 
-	if (update(data)) {
-		/* data copied successfully */
-
-		if (time_topic == 0 || (time_topic != *time)) {
+	if (time_topic == 0 || (time_topic != *time)) {
+		if (orb_copy(_topic, _fd, data) == PX4_OK) {
+			/* data copied successfully */
 			*time = time_topic;
 			return true;
-
-		} else {
-			return false;
 		}
 	}