event: reparent: unmark client from previous frame

Don't just mark it as CLIENT_STALE. To make sure we don't later find the
old frame window as the frame of the client.

Fix assertion failure in ev_destroy_notify.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/event.c

@@ -320,6 +320,7 @@ static inline void ev_reparent_notify(session_t *ps, xcb_reparent_notify_event_t
 	          ev->window, ev->parent, ev->override_redirect);
 	auto w_top = find_toplevel(ps, ev->window);
 	if (w_top) {
+		win_unmark_client(ps, w_top);
 		win_set_flags(w_top, WIN_FLAGS_CLIENT_STALE);
 		ps->pending_updates = true;
 	}