Fix AudioStreamPlaybackMicrophone shutdown crash

[ExpiredPopsicle]

Fixed AudioStreamPlaybackMicrophone rarely attempting to use the AudioServer singleton when it is null, on shutdown.

fixes #116298

[Ivorforce]

I'm not sure this is the right approach to fix this.
I'd expect the AudioServer to be available when AudioStreamPlayback destructs.
Or in other words, I'd expect AudioStreamPlayback instances to destruct before AudioServer destructs.

If we fix this potential error here, but in other stream playbacks, we're leaving the same kind of bug open in other spots as well.

[ExpiredPopsicle]

I've modified the PR to be a more generalized fix, and added a more detailed breakdown of the root cause of the bug in the bug report ( #116298 ).

[Ivorforce]

Thanks! Looks good to me, good sleuthing work!

Reading your explanation, i guess it may actually make sense to also check for AudioServer nullptr in AudioStreamPlayback destruction, just in case someone is incorrectly destructing those after the server is shutdown.

But the fix to clean up lists before clearing the singleton is a good change anyway, and I think we should merge it.

[Repiteo]

Could you squash your commits?

[ExpiredPopsicle]

I was attempting to squash the commits but it looks like I screwed it up.

[Ivorforce]

Try using the following:

git reset --hard 7842b74

7842b74 was your commit before the squash. You should be able to re-attempt a squash afterwards.

[ExpiredPopsicle]

Okay. I managed to squash all the commits down, I think. Sorry about that.

[ExpiredPopsicle]

I'm not sure what's going on with that failed test. Might need a re-run?

[Repiteo]

Thanks!

[Repiteo]

Cherry-picked for 4.6.2.