lavfi/pan: use extended_data instead of data.

Fix trac ticket #3380.

libavfilter/af_pan.c

@@ -370,7 +370,8 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *insamples)
 
     if (!outsamples)
         return AVERROR(ENOMEM);
-    swr_convert(pan->swr, outsamples->data, n, (void *)insamples->data, n);
+    swr_convert(pan->swr, outsamples->extended_data, n,
+                (void *)insamples->extended_data, n);
     av_frame_copy_props(outsamples, insamples);
     outsamples->channel_layout = outlink->channel_layout;
     av_frame_set_channels(outsamples, outlink->channels);