avfilter/af_amix: make independent of the channel layout

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/af_amix.c

@@ -235,7 +235,7 @@ static int config_output(AVFilterLink *outlink)
     if (!s->fifos)
         return AVERROR(ENOMEM);
 
-    s->nb_channels = av_get_channel_layout_nb_channels(outlink->channel_layout);
+    s->nb_channels = outlink->channels;
     for (i = 0; i < s->nb_inputs; i++) {
         s->fifos[i] = av_audio_fifo_alloc(outlink->format, s->nb_channels, 1024);
         if (!s->fifos[i])