[web] Fix possible invalid read during shutdown

The player may emit a listener event when shutting down, and since websockets
didn't remove it's listener callback it would receive an event despite being
deinitialized. This would lead to an invalid read by lws_cancel_service in
listener_cb().

src/websocket.c

@@ -429,6 +429,8 @@ websocket(void *arg)
 #endif
   }
 
+  listener_remove(listener_cb);
+
   pthread_exit(NULL);
 }