ffmpeg: set pkt_timebase

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

ffmpeg.c

@@ -2357,6 +2357,7 @@ static int decode_audio(InputStream *ist, AVPacket *pkt, int *got_output)
     else
         avcodec_get_frame_defaults(ist->decoded_frame);
     decoded_frame = ist->decoded_frame;
+    av_codec_set_pkt_timebase(avctx, ist->st->time_base);
 
     update_benchmark(NULL);
     ret = avcodec_decode_audio4(avctx, decoded_frame, got_output, pkt);
@@ -2476,6 +2477,7 @@ static int decode_video(InputStream *ist, AVPacket *pkt, int *got_output)
         avcodec_get_frame_defaults(ist->decoded_frame);
     decoded_frame = ist->decoded_frame;
     pkt->dts  = av_rescale_q(ist->dts, AV_TIME_BASE_Q, ist->st->time_base);
+    av_codec_set_pkt_timebase(ist->st->codec, ist->st->time_base);
 
     update_benchmark(NULL);
     ret = avcodec_decode_video2(ist->st->codec,