Core: Fix PauseAndLock segfaulting under certain circumstances

[Techjar]

This can happen if it's called before the core has fully initialized, as is happening in https://bugs.dolphin-emu.org/issues/12473.

There might be other side effects from this, so it should probably be tested more thoroughly. Maybe need a mutex?

[CookiePLMonster]

Partially overlaps with #8335, your solution is more localized though.

[Techjar]

Ah, so we both stumbled across the same bug. Didn't realize there was already a PR. However, I did actually find a way to reproduce it without changing code to create a contrived scenario.

In both PRs though, I see the potential for a very tight race condition where RunAsCPUThread can execute non-thread-safe code while the core is still booting, which might be a problem under far more specific circumstances.

[leoetlino]

#8335 is a more complete solution and addresses other problems but I guess it doesn't hurt to fix this more common case first.