ffmpeg: reset dts/pts after decoding the first subpacket

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 6c39b3c278)

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

ffmpeg.c

@@ -2047,6 +2047,10 @@ static int output_packet(InputStream *ist,
 
         if (ret < 0)
             return ret;
+
+        avpkt.dts=
+        avpkt.pts= AV_NOPTS_VALUE;
+
         // touch data and size only if not EOF
         if (pkt) {
             if(ist->st->codec->codec_type != AVMEDIA_TYPE_AUDIO)