avcodec/cdtoons: Fix off by 4 check on diff_size

Fixes: out of array read
Fixes: 20742/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_CDTOONS_fuzzer-5738148607033344

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/cdtoons.c

@@ -269,7 +269,7 @@ static int cdtoons_decode_frame(AVCodecContext *avctx, void *data,
                 diff_size  = bytestream_get_be32(&buf);
                 width      = bytestream_get_be16(&buf);
                 height     = bytestream_get_be16(&buf);
-                if (diff_size < 4 || diff_size - 4 > eod - buf) {
+                if (diff_size < 8 || diff_size - 4 > eod - buf) {
                     av_log(avctx, AV_LOG_WARNING, "Ran (seriously) out of data for Diff frame data.\n");
                     return AVERROR_INVALIDDATA;
                 }