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/uavcan/sensors/gnss.cpp

@@ -48,7 +48,7 @@ const char *const UavcanGnssBridge::NAME = "gnss";
 UavcanGnssBridge::UavcanGnssBridge(uavcan::INode &node) :
 _node(node),
 _sub_fix(node),
-_report_pub(0)
+_report_pub(nullptr)
 {
 }
 
@@ -163,7 +163,7 @@ void UavcanGnssBridge::gnss_fix_sub_cb(const uavcan::ReceivedDataStructure<uavca
 
 	report.satellites_used = msg.sats_used;
 
-	if (_report_pub > 0) {
+	if (_report_pub != nullptr) {
 		orb_publish(ORB_ID(vehicle_gps_position), _report_pub, &report);
 
 	} else {