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/uORB/uORB.h

@@ -135,7 +135,7 @@ __BEGIN_DECLS
  * a file-descriptor-based handle would not otherwise be in scope for the
  * publisher.
  */
-typedef uintptr_t	orb_advert_t;
+typedef void *	orb_advert_t;
 
 /**
  * Advertise as the publisher of a topic.