avcodec/midivid: Check vector index

Fixes: out of array read
Fixes: 20494/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MVDV_fuzzer-5681452423577600

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

libavcodec/midivid.c

@@ -126,6 +126,8 @@ static int decode_mvdv(MidiVidContext *s, AVCodecContext *avctx, AVFrame *frame)
                 idx9bits--;
                 idx = bytestream2_get_byte(gb) | (((idx9val >> (7 - idx9bits)) & 1) << 8);
             }
+            if (idx >= nb_vectors)
+                return AVERROR_INVALIDDATA;
 
             dsty[x  +frame->linesize[0]] = vec[idx * 12 + 0];
             dsty[x+1+frame->linesize[0]] = vec[idx * 12 + 3];