[3.x] Fix crash in AudioServer when switching audio devices with different audio channels count

[Listwon]

Fixes #52275

Switching audio devices with different audio channels count caused crashes in AudioServer::_mix_step() because it tried to access non-existent effects instances in bus->channels[k].effect_instances. AudioServer::init_channels_and_buffers() is called if there is change in get_channel_count(). The update of effects (creating new effects instances) on new channels was missing here.

[akien-mga]

Is this only relevant for 3.x, or is there the same bug in master?

[Listwon]

@akien-mga I've just tried to reproduce it in master build and it's the same bug (easy reproduction with PS5 controller connected/disconnected via USB cable), the same fix works on master too. Should I prepare separate PR for master or you'll apply it yourself when it's tested and merged on 3.x?

[akien-mga]

A separate PR for master would be great. We usually only cherry-pick from newer to older branches, so we don't have a pipeline to keep track of changes the other way around.

[ellenhp]

What happens if the audio thread is actively mixing when this happens? I could imagine we might need to lock the AudioServer in this method before messing with the buffers? LGTM though, because this should fix it most of the time.

[Listwon]

@ellenhp I thought the same initially, but if I'm not mistaken init_channels_and_buffers() is called synchronously before the _mix_step(). Both in the same audio thread, where AudioDriver is already locked before those calls.

[ellenhp]

@akien-mga I've approved both of these and I'll let you either merge both or merge into master and cherry-pick or whatever we want to do.

@Listwon Thanks for figuring this out and fixing it!

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.4.5.