[hwasan] Reset current thread pointer on thread exit.

Summary:
This is necessary to handle calls to free() after __hwasan_thread_exit,
which is possible in glibc.

Also, add a null check to GetCurrentThread, otherwise the logic in
GetThreadByBufferAddress turns it into a non-null value. This means that
all of the checks for GetCurrentThread() != nullptr do not have any
effect at all right now!

Reviewers: pcc, hctim

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D79608