avcodec/vb: Check input packet size to be large enough to contain flags

Fixes: Timeout (->9sec)
Fixes: 16292/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VB_fuzzer-5747063496638464

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vb.c

@@ -199,6 +199,9 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
     uint32_t size;
     int offset = 0;
 
+    if (avpkt->size < 2)
+        return AVERROR_INVALIDDATA;
+
     bytestream2_init(&c->stream, avpkt->data, avpkt->size);
 
     if ((ret = ff_get_buffer(avctx, frame, 0)) < 0)