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/gimbal/gimbal.cpp

@@ -180,7 +180,7 @@ Gimbal::Gimbal() :
 	_attitude_compensation_pitch(true),
 	_attitude_compensation_yaw(true),
 	_initialized(false),
-	_actuator_controls_2_topic(0),
+	_actuator_controls_2_topic(nullptr),
 	_sample_perf(perf_alloc(PC_ELAPSED, "gimbal_read")),
 	_comms_errors(perf_alloc(PC_COUNT, "gimbal_comms_errors")),
 	_buffer_overflows(perf_alloc(PC_COUNT, "gimbal_buffer_overflows"))
@@ -280,7 +280,7 @@ Gimbal::cycle()
 		zero_report.timestamp = hrt_absolute_time();
 		_actuator_controls_2_topic = orb_advertise(ORB_ID(actuator_controls_2), &zero_report);
 
-		if (_actuator_controls_2_topic == 0) {
+		if (_actuator_controls_2_topic == nullptr) {
 			warnx("advert err");
 		}