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/systemlib/param/param.c

@@ -130,7 +130,7 @@ const UT_icd	param_icd = {sizeof(struct param_wbuf_s), NULL, NULL, NULL};
 ORB_DEFINE(parameter_update, struct parameter_update_s);
 
 /** parameter update topic handle */
-static orb_advert_t param_topic = 0;
+static orb_advert_t param_topic = NULL;
 
 static void param_set_used_internal(param_t param);
 
@@ -233,7 +233,7 @@ param_notify_changes(void)
 	 * If we don't have a handle to our topic, create one now; otherwise
 	 * just publish.
 	 */
-	if (param_topic == 0) {
+	if (param_topic == NULL) {
 		param_topic = orb_advertise(ORB_ID(parameter_update), &pup);
 
 	} else {