fftools/ffmpeg: Fix integer overflow in duration computation in seek_to_start()

Fixes: signed integer overflow: -9223372036854775808 - 9223372036854775807 cannot be represented in type 'long'
Fixes: Ticket8142

Found-by: Suhwan
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

fftools/ffmpeg.c

@@ -4225,6 +4225,7 @@ static int seek_to_start(InputFile *ifile, AVFormatContext *is)
             ifile->time_base = ist->st->time_base;
         /* the total duration of the stream, max_pts - min_pts is
          * the duration of the stream without the last frame */
+        if (ist->max_pts > ist->min_pts && ist->max_pts - (uint64_t)ist->min_pts < INT64_MAX - duration)
             duration += ist->max_pts - ist->min_pts;
         ifile->time_base = duration_max(duration, &ifile->duration, ist->st->time_base,
                                         ifile->time_base);