vd_lavc: use avcodec_is_open()

This is "better", although nobody seems to know how this API is supposed
to work at all.

video/decode/vd_lavc.c

@@ -430,7 +430,7 @@ static void uninit_avctx(struct dec_video *vd)
     vd_ffmpeg_ctx *ctx = vd->priv;
     AVCodecContext *avctx = ctx->avctx;
 
-    if (avctx && avctx->codec)
+    if (avctx && avcodec_is_open(avctx))
         avcodec_flush_buffers(avctx);
 
     if (ctx->hwdec && ctx->hwdec->uninit)