avformat/utils: Fix integer overflow of fps_first/last_dts

Fixes: runtime error: signed integer overflow: 7738135736989908991 - -7898362169240453118 cannot be represented in type 'long'
Fixes: Chromium bug 796778
Reported-by: Matt Wolenetz <wolenetz@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/utils.c

@@ -3778,7 +3778,7 @@ FF_ENABLE_DEPRECATION_WARNINGS
             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 >
-                (st->info->fps_last_dts     - st->info->fps_first_dts) /
+                (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,
                        "DTS discontinuity in stream %d: packet %d with DTS "