ffmpeg: Check dts before use in non monotone dts workaround

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -612,6 +612,7 @@ static void write_frame(AVFormatContext *s, AVPacket *pkt, OutputStream *ost)
 
     if (!(s->oformat->flags & AVFMT_NOTIMESTAMPS) &&
         (avctx->codec_type == AVMEDIA_TYPE_AUDIO || avctx->codec_type == AVMEDIA_TYPE_VIDEO) &&
+        pkt->dts != AV_NOPTS_VALUE &&
         ost->last_mux_dts != AV_NOPTS_VALUE &&
         pkt->dts < ost->last_mux_dts + !(s->oformat->flags & AVFMT_TS_NONSTRICT)) {
         av_log(NULL, AV_LOG_WARNING, "Non-monotonous DTS in output stream "