Clear window alert flags after setting winlink alert flags, fixes problem

reported by Tommy Allen.

alerts.c

@@ -62,6 +62,8 @@ alerts_callback(__unused int fd, __unused short events, __unused void *arg)
 
 		w->alerts_queued = 0;
 		TAILQ_REMOVE(&alerts_list, w, alerts_entry);
+
+		w->flags &= ~WINDOW_ALERTFLAGS;
 		window_remove_ref(w);
 	}
 	alerts_fired = 0;