lavfi/volume: use copy props helper instead of incomplete manual code.

libavfilter/af_volume.c

@@ -234,7 +234,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *buf)
         out_buf = ff_get_audio_buffer(inlink, nb_samples);
         if (!out_buf)
             return AVERROR(ENOMEM);
-        out_buf->pts = buf->pts;
+        av_frame_copy_props(out_buf, buf);
     }
 
     if (vol->precision != PRECISION_FIXED || vol->volume_i > 0) {