avcodec/bfi: use av_freep(), do not leave stale pointers in memory

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

libavcodec/bfi.c

@@ -171,7 +171,7 @@ static int bfi_decode_frame(AVCodecContext *avctx, void *data,
 static av_cold int bfi_decode_close(AVCodecContext *avctx)
 {
     BFIContext *bfi = avctx->priv_data;
-    av_free(bfi->dst);
+    av_freep(&bfi->dst);
     return 0;
 }