avcodec/cbs_vp9: Check index_size

Fixes: out of array read
Fixes: 19300/clusterfuzz-testcase-minimized-ffmpeg_BSF_VP9_METADATA_fuzzer-5653911730126848

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

libavcodec/cbs_vp9.c

@@ -428,6 +428,9 @@ static int cbs_vp9_split_fragment(CodedBitstreamContext *ctx,
         index_size = 2 + (((superframe_header & 0x18) >> 3) + 1) *
                           ((superframe_header & 0x07) + 1);
 
+        if (index_size > frag->data_size)
+            return AVERROR_INVALIDDATA;
+
         err = init_get_bits(&gbc, frag->data + frag->data_size - index_size,
                             8 * index_size);
         if (err < 0)