avformat/mov: compute absolute dts difference without overflow in mov_find_next_sample()

Fixes: signed integer overflow: -9223372036854775808 - 9222726413022000000 cannot be represented in type 'long'
Fixes: 51896/clusterfuzz-testcase-minimized-ffmpeg_dem_MOV_fuzzer-5959420033761280

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 3508b496e1)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -8654,12 +8654,13 @@ static AVIndexEntry *mov_find_next_sample(AVFormatContext *s, AVStream **st)
         if (msc->pb && msc->current_sample < avsti->nb_index_entries) {
             AVIndexEntry *current_sample = &avsti->index_entries[msc->current_sample];
             int64_t dts = av_rescale(current_sample->timestamp, AV_TIME_BASE, msc->time_scale);
+            uint64_t dtsdiff = best_dts > dts ? best_dts - (uint64_t)dts : ((uint64_t)dts - best_dts);
             av_log(s, AV_LOG_TRACE, "stream %d, sample %d, dts %"PRId64"\n", i, msc->current_sample, dts);
             if (!sample || (!(s->pb->seekable & AVIO_SEEKABLE_NORMAL) && current_sample->pos < sample->pos) ||
                 ((s->pb->seekable & AVIO_SEEKABLE_NORMAL) &&
                  ((msc->pb != s->pb && dts < best_dts) || (msc->pb == s->pb && dts != AV_NOPTS_VALUE &&
-                 ((FFABS(best_dts - dts) <= AV_TIME_BASE && current_sample->pos < sample->pos) ||
-                  (FFABS(best_dts - dts) > AV_TIME_BASE && dts < best_dts)))))) {
+                 ((dtsdiff <= AV_TIME_BASE && current_sample->pos < sample->pos) ||
+                  (dtsdiff > AV_TIME_BASE && dts < best_dts)))))) {
                 sample = current_sample;
                 best_dts = dts;
                 *st = avst;