avformat/utils: check dts/duration to be representable before using them

Fixes: signed integer overflow: 6854513951393103890 + 3427256975738527712 cannot be represented in type 'long'
Fixes: 32936/clusterfuzz-testcase-minimized-ffmpeg_dem_R3D_fuzzer-5236914752978944

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

@@ -1157,7 +1157,9 @@ static void update_initial_durations(AVFormatContext *s, AVStream *st,
             (pktl->pkt.dts == AV_NOPTS_VALUE ||
              pktl->pkt.dts == st->internal->first_dts ||
              pktl->pkt.dts == RELATIVE_TS_BASE) &&
-            !pktl->pkt.duration) {
+            !pktl->pkt.duration &&
+            av_sat_add64(cur_dts, duration) == cur_dts + (uint64_t)duration
+        ) {
             pktl->pkt.dts = cur_dts;
             if (!st->internal->avctx->has_b_frames)
                 pktl->pkt.pts = cur_dts;