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/navigator/geofence.cpp

@@ -69,7 +69,7 @@ static const int ERROR = -1;
 
 Geofence::Geofence() :
 	SuperBlock(NULL, "GF"),
-	_fence_pub(0),
+	_fence_pub(nullptr),
 	_home_pos{},
 	_home_pos_set(false),
 	_last_horizontal_range_warning(0),
@@ -317,7 +317,7 @@ Geofence::addPoint(int argc, char *argv[])
 void
 Geofence::publishFence(unsigned vertices)
 {
-	if (_fence_pub == 0) {
+	if (_fence_pub == nullptr) {
 		_fence_pub = orb_advertise(ORB_ID(fence), &vertices);
 
 	} else {