avformat/utils: Avoid overflow in codec_info_duration computation for subtitles

Fixes: signed integer overflow: 9223126845747118112 - -2594073385365397472 cannot be represented in type 'long'
Fixes: 34936/clusterfuzz-testcase-minimized-ffmpeg_dem_MATROSKA_fuzzer-6739888002170880

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

libavformat/utils.c

@@ -3843,7 +3843,9 @@ int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options)
                 break;
             }
             if (pkt->duration) {
-                if (avctx->codec_type == AVMEDIA_TYPE_SUBTITLE && pkt->pts != AV_NOPTS_VALUE && st->start_time != AV_NOPTS_VALUE && pkt->pts >= st->start_time) {
+                if (avctx->codec_type == AVMEDIA_TYPE_SUBTITLE && pkt->pts != AV_NOPTS_VALUE && st->start_time != AV_NOPTS_VALUE && pkt->pts >= st->start_time
+                    && (uint64_t)pkt->pts - st->start_time < INT64_MAX
+                ) {
                     st->internal->info->codec_info_duration = FFMIN(pkt->pts - st->start_time, st->internal->info->codec_info_duration + pkt->duration);
                 } else
                     st->internal->info->codec_info_duration += pkt->duration;