lavc: update pkt_duration for skipped samples.

Also: factor the the computation of the timestamp difference.

libavcodec/utils.c

@@ -1664,10 +1664,15 @@ int attribute_align_arg avcodec_decode_audio4(AVCodecContext *avctx,
                 av_samples_copy(frame->extended_data, frame->extended_data, 0, avctx->internal->skip_samples,
                                 frame->nb_samples - avctx->internal->skip_samples, avctx->channels, frame->format);
                 if(avctx->pkt_timebase.num && avctx->sample_rate) {
+                    int64_t diff_ts = av_rescale_q(avctx->internal->skip_samples,
+                                                   (AVRational){1, avctx->sample_rate},
+                                                   avctx->pkt_timebase);
                     if(frame->pkt_pts!=AV_NOPTS_VALUE)
-                        frame->pkt_pts += av_rescale_q(avctx->internal->skip_samples,(AVRational){1, avctx->sample_rate}, avctx->pkt_timebase);
+                        frame->pkt_pts += diff_ts;
                     if(frame->pkt_dts!=AV_NOPTS_VALUE)
-                        frame->pkt_dts += av_rescale_q(avctx->internal->skip_samples,(AVRational){1, avctx->sample_rate}, avctx->pkt_timebase);
+                        frame->pkt_dts += diff_ts;
+                    if (frame->pkt_duration >= diff_ts)
+                        frame->pkt_duration -= diff_ts;
                 } else {
                     av_log(avctx, AV_LOG_WARNING, "Could not update timestamps for skipped samples.\n");
                 }