cook: Make sure there is enough extradata

At least 8 bytes are needed (Mono audio).

Bug-Id: CID 741418
CC: libav-stable@libav.org

libavcodec/cook.c

@@ -1051,7 +1051,7 @@ static av_cold int cook_decode_init(AVCodecContext *avctx)
     q->avctx = avctx;
 
     /* Take care of the codec specific extradata. */
-    if (extradata_size <= 0) {
+    if (extradata_size < 8) {
         av_log(avctx, AV_LOG_ERROR, "Necessary extradata missing!\n");
         return AVERROR_INVALIDDATA;
     }