check client is mapped before setting border color

For some reason brave configured for as a wayland client triggers this code on startup and segfaults.

Checking if the client is mapped fixes this, like with the previous fix for urgent border colour.

References: 887fde65a3
Fixes: 72a7d78a1a

dwl.c

@@ -2593,6 +2593,7 @@ urgent(struct wl_listener *listener, void *data)
 	if (!c || c == focustop(selmon))
 		return;
 
+	if (client_is_mapped(c))
 		client_set_border_color(c, urgentcolor);
 	c->isurgent = 1;
 	printstatus();