orb_advert_t changed to void * and checks changed to nullptr

The existing orb_advert_t use thoughout the code sometimes tries
to treat it as a file descriptor and there are checks for < 0
and ::close calls on orb_advert_t types which is an invalid use
of an object pointer, which is what orb_advert_t really is.

Initially I had changed the -1 initializations to 0 but it was
suggested that this should be nullptr. That was a good recommendation
but the definition of orb_advert_t had to change to void * because
you cannot initialize a uintptr_t as nullptr.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/modules/land_detector/LandDetector.cpp

@@ -72,7 +72,7 @@ void LandDetector::start()
 	// advertise the first land detected uORB
 	_landDetected.timestamp = hrt_absolute_time();
 	_landDetected.landed = false;
-	_landDetectedPub = orb_advertise(ORB_ID(vehicle_land_detected), &_landDetected);
+	_landDetectedPub = (uintptr_t)orb_advertise(ORB_ID(vehicle_land_detected), &_landDetected);
 
 	// initialize land detection algorithm
 	initialize();
@@ -91,7 +91,7 @@ void LandDetector::start()
 			_landDetected.landed = landDetected;
 
 			// publish the land detected broadcast
-			orb_publish(ORB_ID(vehicle_land_detected), _landDetectedPub, &_landDetected);
+			orb_publish(ORB_ID(vehicle_land_detected), (orb_advert_t)_landDetectedPub, &_landDetected);
 		}
 
 		// limit loop rate

src/modules/land_detector/LandDetector.h

@@ -93,7 +93,7 @@ protected:
                                                                           before triggering a land */
 
 protected:
-	orb_advert_t                            _landDetectedPub;           /**< publisher for position in local frame */
+	uintptr_t                               _landDetectedPub;           /**< publisher for position in local frame */
 	struct vehicle_land_detected_s          _landDetected;              /**< local vehicle position */
 
 private: