binkaudio: prevent extradata overread

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/binkaudio.c

@@ -96,7 +96,7 @@ static av_cold int decode_init(AVCodecContext *avctx)
         return AVERROR_INVALIDDATA;
     }
 
-    s->version_b = avctx->extradata && avctx->extradata[3] == 'b';
+    s->version_b = avctx->extradata_size >= 4 && avctx->extradata[3] == 'b';
 
     if (avctx->codec->id == CODEC_ID_BINKAUDIO_RDFT) {
         // audio is already interleaved for the RDFT format variant