avformat/mov: Fix integer overflow in mov_read_packet().

Fixes https://crbug.com/1499669:
runtime error: signed integer overflow: 9223372036853334272 + 1375731456
cannot be represented in type 'int64_t' (aka 'long')

Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 2182173a69)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -8861,7 +8861,7 @@ static int mov_read_packet(AVFormatContext *s, AVPacket *pkt)
         pkt->flags |= AV_PKT_FLAG_DISCARD;
     }
     if (sc->ctts_data && sc->ctts_index < sc->ctts_count) {
-        pkt->pts = pkt->dts + sc->dts_shift + sc->ctts_data[sc->ctts_index].duration;
+        pkt->pts = av_sat_add64(pkt->dts, av_sat_add64(sc->dts_shift, sc->ctts_data[sc->ctts_index].duration));
         /* update ctts context */
         sc->ctts_sample++;
         if (sc->ctts_index < sc->ctts_count &&