player/video: set video_out to NULL before broadcasting events

This avoids possible reference of video_out after destory.

player/video.c

@@ -129,9 +129,9 @@ void uninit_video_out(struct MPContext *mpctx)
     uninit_video_chain(mpctx);
     if (mpctx->video_out) {
         vo_destroy(mpctx->video_out);
+        mpctx->video_out = NULL;
         mp_notify(mpctx, MPV_EVENT_VIDEO_RECONFIG, NULL);
     }
-    mpctx->video_out = NULL;
 }
 
 static void vo_chain_uninit(struct vo_chain *vo_c)