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/px4fmu/fmu.cpp

@@ -253,7 +253,7 @@ PX4FMU::PX4FMU() :
 	_current_update_rate(0),
 	_task(-1),
 	_armed_sub(-1),
-	_outputs_pub(-1),
+	_outputs_pub(nullptr),
 	_armed{},
 	_num_outputs(0),
 	_class_instance(0),
@@ -692,7 +692,7 @@ PX4FMU::task_main()
 				}
 
 				/* publish mixed control outputs */
-				if (_outputs_pub < 0) {
+				if (_outputs_pub != nullptr) {
 					_outputs_pub = orb_advertise_multi(ORB_ID(actuator_outputs), &outputs, &_actuator_output_topic_instance, ORB_PRIO_DEFAULT);
 				} else {
 
@@ -1837,7 +1837,7 @@ fake(int argc, char *argv[])
 
 	orb_advert_t handle = orb_advertise(ORB_ID_VEHICLE_ATTITUDE_CONTROLS, &ac);
 
-	if (handle < 0)
+	if (handle == nullptr)
 		errx(1, "advertise failed");
 
 	actuator_armed_s aa;
@@ -1847,7 +1847,7 @@ fake(int argc, char *argv[])
 
 	handle = orb_advertise(ORB_ID(actuator_armed), &aa);
 
-	if (handle < 0)
+	if (handle == nullptr)
 		errx(1, "advertise failed 2");
 
 	exit(0);