opt: make sure data is valid when swallowing window come back

maomao.c

@@ -1350,6 +1350,7 @@ swallow(Client *c, Client *w)
   c->ismaxmizescreen = w->ismaxmizescreen;
 	c->tags = w->tags;
 	c->geom = w->geom;
+  c->mon = w->mon;
   c->scroller_proportion = w->scroller_proportion;
 	wl_list_insert(&w->link, &c->link);
 	wl_list_insert(&w->flink, &c->flink);