avformat/mov: Only set pkt->duration to non negative values

Reviewed-by: Sasi Inguva <isasi@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -7586,7 +7586,9 @@ static int mov_read_packet(AVFormatContext *s, AVPacket *pkt)
     } else {
         int64_t next_dts = (sc->current_sample < st->nb_index_entries) ?
             st->index_entries[sc->current_sample].timestamp : st->duration;
-        pkt->duration = next_dts - pkt->dts;
+
+        if (next_dts >= pkt->dts)
+            pkt->duration = next_dts - pkt->dts;
         pkt->pts = pkt->dts;
     }
     if (st->discard == AVDISCARD_ALL)