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/hott/messages.cpp

@@ -63,7 +63,7 @@ static int _sensor_sub = -1;
 static int _airspeed_sub = -1;
 static int _esc_sub = -1;
 
-static orb_advert_t _esc_pub = 0;
+static orb_advert_t _esc_pub = nullptr;
 
 static bool _home_position_set = false;
 static double _home_lat = 0.0d;
@@ -120,7 +120,7 @@ publish_gam_message(const uint8_t *buffer)
 	esc.esc[0].esc_current = static_cast<float>((msg.current_H << 8) | (msg.current_L & 0xff)) * 0.1F;
 
 	/* announce the esc if needed, just publish else */
-	if (_esc_pub > 0) {
+	if (_esc_pub != nullptr) {
 		orb_publish(ORB_ID(esc_status), _esc_pub, &esc);
 	} else {
 		_esc_pub = orb_advertise(ORB_ID(esc_status), &esc);