lavfi/af_amerge: set outbuf->audio->channels.

The value is lost because of avfilter_copy_buffer_ref_props().

Fix trac ticket #2217.

libavfilter/af_amerge.c

@@ -260,6 +260,7 @@ static int filter_frame(AVFilterLink *inlink, AVFilterBufferRef *insamples)
 
     outbuf->audio->nb_samples     = nb_samples;
     outbuf->audio->channel_layout = outlink->channel_layout;
+    outbuf->audio->channels       = outlink->channels;
 
     while (nb_samples) {
         ns = nb_samples;