avutil/frame: fix av_frame_copy for unknown layouts

I wonder how unknown layouts ever worked without this?
Reviewed-by: Nicolas George <george@nsup.org>
Reviewed-by: Hendrik Leppkes <h.leppkes@gmail.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Marton Balint <cus@passwd.hu>

libavutil/frame.c

@@ -725,7 +725,7 @@ int av_frame_copy(AVFrame *dst, const AVFrame *src)
 
     if (dst->width > 0 && dst->height > 0)
         return frame_copy_video(dst, src);
-    else if (dst->nb_samples > 0 && dst->channel_layout)
+    else if (dst->nb_samples > 0 && dst->channels > 0)
         return frame_copy_audio(dst, src);
 
     return AVERROR(EINVAL);