af_join: fix channel count and format

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/af_join.c

@@ -476,6 +476,8 @@ static int join_request_frame(AVFilterLink *outlink)
 
     frame->nb_samples     = nb_samples;
     frame->channel_layout = outlink->channel_layout;
+    av_frame_set_channels(frame, outlink->channels);
+    frame->format         = outlink->format;
     frame->sample_rate    = outlink->sample_rate;
     frame->pts            = s->input_frames[0]->pts;
     frame->linesize[0]    = linesize;