ao_coreaudio: error out when selecting invalid device

When selecting a device that simply doesn't exist with --audio-device,
AudioUnit will still initialize and start playback without complaining.
But it will never call the audio render callback, which leads to audio
playback simply not progressing.

I couldn't find a way to get AudioUnit to report an error at all, so
here's a crappy hack that takes care of this in most cases. We assume
that all devices have a kAudioDevicePropertyDeviceIsAlive property.
Invalid devices will error when querying the property (with 'obj!' as
status code).

This is not the correct fix, because we try to double-guess AudioUnit's
behavior by accessing a lower label API. Suggestions welcome.

audio/out/ao_coreaudio_utils.c

@@ -114,6 +114,13 @@ OSStatus ca_select_device(struct ao *ao, char* name, AudioDeviceID *device)
             kAudioObjectSystemObject, &p_addr, 0, 0, &size, &v);
         CFRelease(uid);
         CHECK_CA_ERROR("unable to query for device UID");
+
+        uint32_t is_alive = 1;
+        err = CA_GET(*device, kAudioDevicePropertyDeviceIsAlive, &is_alive);
+        CHECK_CA_ERROR("could not check whether device is alive (invalid device?)");
+
+        if (!is_alive)
+            MP_WARN(ao, "device is not alive!\n");
     } else {
         // device not set by user, get the default one
         err = CA_GET(kAudioObjectSystemObject,