avcodec/ffv1dec: Test extradata_size instead of extradata for better robustness

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ffv1dec.c

@@ -881,7 +881,7 @@ static av_cold int decode_init(AVCodecContext *avctx)
     if ((ret = ff_ffv1_common_init(avctx)) < 0)
         return ret;
 
-    if (avctx->extradata && (ret = read_extra_header(f)) < 0)
+    if (avctx->extradata_size > 0 && (ret = read_extra_header(f)) < 0)
         return ret;
 
     if ((ret = ff_ffv1_init_slice_contexts(f)) < 0)