avformat/utils: Fix integer overflow in discontinuity check

Fixes: signed integer overflow: 7738135736989908991 - -7954308516317364223 cannot be represented in type 'long'
Fixes: find_stream_info_usan

Reported-by: Thomas Guilbert <tguilbert@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 4e19cfcfa3944fe4cf97bea758f72f104dcaebad)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/utils.c

@@ -3326,7 +3326,7 @@ int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options)
              * sequence, we treat it as a discontinuity. */
             if (st->info->fps_last_dts != AV_NOPTS_VALUE &&
                 st->info->fps_last_dts_idx > st->info->fps_first_dts_idx &&
-                (pkt->dts - st->info->fps_last_dts) / 1000 >
+                (pkt->dts - (uint64_t)st->info->fps_last_dts) / 1000 >
                 (st->info->fps_last_dts     - (uint64_t)st->info->fps_first_dts) /
                 (st->info->fps_last_dts_idx - st->info->fps_first_dts_idx)) {
                 av_log(ic, AV_LOG_WARNING,