libavcodec/libxavs: Use av_freep() avoid leaving stale pointers in memory

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/libxavs.c

@@ -201,7 +201,7 @@ static av_cold int XAVS_close(AVCodecContext *avctx)
     XavsContext *x4 = avctx->priv_data;
 
     av_freep(&avctx->extradata);
-    av_free(x4->sei);
+    av_freep(&x4->sei);
     av_freep(&x4->pts_buffer);
 
     if (x4->enc)