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/commander/state_machine_helper.cpp

@@ -482,7 +482,7 @@ static transition_result_t disable_publication(const int mavlink_fd)
 /**
  * Transition from one hil state to another
  */
-transition_result_t hil_state_transition(hil_state_t new_state, int status_pub, struct vehicle_status_s *current_status, const int mavlink_fd)
+transition_result_t hil_state_transition(hil_state_t new_state, orb_advert_t status_pub, struct vehicle_status_s *current_status, const int mavlink_fd)
 {
 	transition_result_t ret = TRANSITION_DENIED;