[compiler-rt] [sanitizer] Remove GetCurrentThread nullness checks from Allocate (PR #102828)

[Vitaly Buka via llvm-commits]

vitalybuka wrote:

> The `*Allocate` functions must be called after `*_current_thread` is set. Delete the nullness checks.

With static runtime.

With dynamic runtime, with late activation, it's not guaranteed, I guess?

https://github.com/llvm/llvm-project/pull/102828