avcodec/utils: fix packet duration of frames with discarded paddings

Reviewed-by: Hendrik Leppkes <h.leppkes@gmail.com>
Signed-off-by: Marton Balint <cus@passwd.hu>

libavcodec/utils.c

@@ -2337,8 +2337,7 @@ int attribute_align_arg avcodec_decode_audio4(AVCodecContext *avctx,
                     int64_t diff_ts = av_rescale_q(frame->nb_samples - discard_padding,
                                                    (AVRational){1, avctx->sample_rate},
                                                    avctx->pkt_timebase);
-                    if (av_frame_get_pkt_duration(frame) >= diff_ts)
-                        av_frame_set_pkt_duration(frame, av_frame_get_pkt_duration(frame) - diff_ts);
+                    av_frame_set_pkt_duration(frame, diff_ts);
                 } else {
                     av_log(avctx, AV_LOG_WARNING, "Could not update timestamps for discarded samples.\n");
                 }