avcodec/shorten: Check verbatim length

Fixes: Timeout
Fixes: 9252/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SHORTEN_fuzzer-5780720709533696

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 7007dabec0)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/shorten.c

@@ -518,6 +518,11 @@ static int shorten_decode_frame(AVCodecContext *avctx, void *data,
             switch (cmd) {
             case FN_VERBATIM:
                 len = get_ur_golomb_shorten(&s->gb, VERBATIM_CKSIZE_SIZE);
+                if (len < 0 || len > get_bits_left(&s->gb)) {
+                    av_log(avctx, AV_LOG_ERROR, "verbatim length %d invalid\n",
+                           len);
+                    return AVERROR_INVALIDDATA;
+                }
                 while (len--)
                     get_ur_golomb_shorten(&s->gb, VERBATIM_BYTE_SIZE);
                 break;