correctly handle interval, call callback only when topic updated, add example for 2 topics

src/platforms/px4_nodehandle.h

@@ -114,7 +114,7 @@ public:
 		SubscriberPX4<M> *sub_px4 = new SubscriberPX4<M>(meta, interval, callback, &_subs);
 
 		/* Check if this is the smallest interval so far and update _sub_min_interval */
-		if (_sub_min_interval == nullptr || _sub_min_interval->getInterval() < sub_px4->getInterval()) {
+		if (_sub_min_interval == nullptr || _sub_min_interval->getInterval() > sub_px4->getInterval()) {
 			_sub_min_interval = sub_px4;
 		}
 		return (Subscriber*)sub_px4;

src/platforms/px4_subscriber.h

@@ -89,6 +89,11 @@ public:
 	~SubscriberPX4() {};
 
 	void update() {
+		if (!uORB::Subscription<M>::updated()) {
+			/* Topic not updated, do not call callback */
+			return;
+		}
+
 		/* get latest data */
 		uORB::Subscription<M>::update();