music-control - null out dead timer so it isnt accidentally deled

deleting invalid timer object... because it is invalid because we
never nulled out the timer when it runs.

src/modules/music-control/e_mod_main.c

@@ -417,6 +417,7 @@ static Eina_Bool
 _poll_dbus_cb(void *data)
 {
    E_Music_Control_Module_Context *ctxt = data;
+   ctxt->poll_timer = NULL;
    _bus_list(ctxt);
    return EINA_FALSE;
 }