audio: unregister callback

This caused occasional crashes on M1 Mac Pro with `uv --capabilities`
(also in macos-14 arm64 GH runner). This was most likely caused by the
callback being run just after the audio state has been destroyed.

src/audio/audio.cpp

@@ -561,6 +561,9 @@ void audio_done(struct state_audio *s)
 
         audio_codec_done(s->audio_encoder);
 
+        unregister_should_exit_callback(get_root_module(s->mod.get()),
+                                        should_exit_audio, s);
+
         delete s;
 }