core: Ensure presencestate subtype and message are NULL.

When retrieving presence state information there is no
guarantee that the subtype and message passed in are
set to NULL. This change ensures they are.

ASTERISK-26397 #close

Change-Id: I61f8187972d5d8bbd7d6b7f4daa4f4f7e8237b23

main/presencestate.c

@@ -151,6 +151,9 @@ static enum ast_presence_state ast_presence_state_helper(const char *presence_pr
 	char *label = ast_strdupa(presence_provider);
 	int res = AST_PRESENCE_INVALID;
 
+	*subtype = NULL;
+	*message = NULL;
+
 	if (check_cache) {
 		res = presence_state_cached(presence_provider, subtype, message);
 		if (res != AST_PRESENCE_INVALID) {