Fix Issue 23350 - Nondeterministic test failure in std.concurrency

[pbackus]

Previously, a TOCTOU bug in threadInfo could result in a null dereference if 'scheduler' was changed to null between the evaluation of the 'if' condition and the call to scheduler.threadInfo.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	23350	normal	Nondeterministic test failure in std.concurrency

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8579"

[pbackus]

Technically this is still a data race unless all accesses to scheduler are made atomic, but it seems like that's something we can get away with on the platforms we currently test on, so probably ok to put off fixing it for now.