res_sorcery_realtime.c: Fix crash from NULL sorcery object type.

If the sorcery object type is not found a NULL is returned.
Unfortunately, sorcery_realtime_filter_objectset() will crash after
complaining about not finding the object type and saying to expect errors.

* Use ao2_cleanup() instead of ao2_ref() to prevent the crash.

ASTERISK-25165
Reported by Corey Farrell

Change-Id: Ic3b64453ea3058cb68d5c26d97d4fe7b8eea2e97

res/res_sorcery_realtime.c

@@ -142,7 +142,7 @@ static struct ast_variable *sorcery_realtime_filter_objectset(struct ast_variabl
 		}
 	}
 
-	ao2_ref(object_type, -1);
+	ao2_cleanup(object_type);
 
 	return objectset;
 }