udpsrc: don't crash dereferencing NULL error when leaving multicast group on shutdown

Strangely enough, if we do pass an error variable to be filled, we
no longer get an error on leaving.

gst/udp/gstudpsrc.c

@@ -954,7 +954,7 @@ gst_udpsrc_stop (GstBaseSrc * bsrc)
 
       if (!g_socket_leave_multicast_group (src->used_socket,
               g_inet_socket_address_get_address (src->addr), FALSE,
-              src->multi_iface, NULL)) {
+              src->multi_iface, &err)) {
         GST_ERROR_OBJECT (src, "Failed to leave multicast group: %s",
             err->message);
         g_clear_error (&err);