ao_coreaudio: do not accept unknown channel layouts

Coreaudio gives us a channel map with all entries set to
kAudioChannelLabel_Unknown. This is translated to a mpv channel map with
all channels set to NA, which has special meaning: it's an "unknown"
channel map, which acts as wildcard and can be converted from/to any
channel layout. Not really what we want.

I've got this with USB audio, playing stereo. The multichannel layout
consisted of 2 unknown channels, while the stereo channel map was
stereo (as expected).

Note that channel maps with _some_ NA entries are not affected by this,
and must still work.

audio/out/ao_coreaudio_chmap.c

@@ -164,7 +164,7 @@ static bool ca_layout_to_mp_chmap(struct ao *ao, AudioChannelLayout *layout,
     }
 
     talloc_free(talloc_ctx);
-    return chmap->num > 0;
+    return mp_chmap_is_valid(chmap) && !mp_chmap_is_unknown(chmap);
 coreaudio_error:
     MP_VERBOSE(ao, "converted input channel layout (failed):\n");
     ca_log_layout(ao, MSGL_V, layout);