ffplay: Fix usage of private lavfi API

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>

ffplay.c

@@ -2725,7 +2725,7 @@ static int stream_component_open(VideoState *is, int stream_index)
                 goto fail;
             link = is->out_audio_filter->inputs[0];
             sample_rate    = link->sample_rate;
-            nb_channels    = link->channels;
+            nb_channels    = avfilter_link_get_channels(link);
             channel_layout = link->channel_layout;
         }
 #else