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/batt_smbus/batt_smbus.cpp

@@ -203,7 +203,7 @@ BATT_SMBUS::BATT_SMBUS(int bus, uint16_t batt_smbus_addr) :
 	_enabled(false),
 	_work{},
 	_reports(nullptr),
-	_batt_topic(0),
+	_batt_topic(nullptr),
 	_batt_orb_id(nullptr),
 	_start_time(0),
 	_batt_capacity(0)
@@ -427,13 +427,13 @@ BATT_SMBUS::cycle()
 		}
 
 		// publish to orb
-		if (_batt_topic != 0) {
+		if (_batt_topic != nullptr) {
 			orb_publish(_batt_orb_id, _batt_topic, &new_report);
 
 		} else {
 			_batt_topic = orb_advertise(_batt_orb_id, &new_report);
 
-			if (_batt_topic == 0) {
+			if (_batt_topic == nullptr) {
 				errx(1, "ADVERT FAIL");
 			}
 		}