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/drivers/sf0x/sf0x.cpp

@@ -293,7 +293,6 @@ SF0X::init()
 
 		_class_instance = register_class_devname(RANGE_FINDER_BASE_DEVICE_PATH);
 
-<<<<<<< HEAD
 		if (_class_instance == CLASS_DEVICE_PRIMARY) {
 			/* get a publish handle on the range finder topic */
 			struct distance_sensor_s ds_report = {};
@@ -301,13 +300,9 @@ SF0X::init()
 			_distance_sensor_topic = orb_advertise_multi(ORB_ID(distance_sensor), &ds_report,
 								     &_orb_class_instance, ORB_PRIO_HIGH);
 
-			if (_distance_sensor_topic < 0) {
+			if (_distance_sensor_topic == nullptr) {
 				log("failed to create distance_sensor object. Did you start uOrb?");
 			}
-=======
-		if (_range_finder_topic == 0) {
-			warnx("advert err");
->>>>>>> extensive orb_advert_t fixes
 		}
 
 	} while(0);