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/simulator/simulator.h

@@ -165,10 +165,10 @@ private:
 	_accel(1),
 	_mpu(1),
 	_baro(1),
-	_sensor_combined_pub(-1)
+	_sensor_combined_pub(nullptr)
 #ifndef __PX4_QURT
 	,
-	_manual_control_sp_pub(-1),
+	_manual_control_sp_pub(nullptr),
 	_actuator_outputs_sub(-1),
 	_vehicle_attitude_sub(-1),
 	_sensor{},