avfilter/audio: Don't call av_get_channel_layout_nb_channels twice

Also makes the assert-string shorter.
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/audio.c

@@ -40,8 +40,9 @@ AVFrame *ff_default_get_audio_buffer(AVFilterLink *link, int nb_samples)
 {
     AVFrame *frame = NULL;
     int channels = link->channels;
+    int channel_layout_nb_channels = av_get_channel_layout_nb_channels(link->channel_layout);
 
-    av_assert0(channels == av_get_channel_layout_nb_channels(link->channel_layout) || !av_get_channel_layout_nb_channels(link->channel_layout));
+    av_assert0(channels == channel_layout_nb_channels || !channel_layout_nb_channels);
 
     if (!link->frame_pool) {
         link->frame_pool = ff_frame_pool_audio_init(av_buffer_allocz, channels,