lavc: set best_effort_timestamp for audio too.

libavcodec/utils.c

@@ -1640,6 +1640,9 @@ int attribute_align_arg avcodec_decode_audio4(AVCodecContext *avctx,
         if (ret >= 0 && *got_frame_ptr) {
             avctx->frame_number++;
             frame->pkt_dts = avpkt->dts;
+            frame->best_effort_timestamp = guess_correct_pts(avctx,
+                                                             frame->pkt_pts,
+                                                             frame->pkt_dts);
             if (frame->format == AV_SAMPLE_FMT_NONE)
                 frame->format = avctx->sample_fmt;
             if (!frame->channel_layout)