Avoid a race in init_thread_stop()

Avoid a race in init_thread_stop()

init_thread_stop() is called when a thread is stopping. It calls all
the callbacks that need to know about the demise of this thread. However,
the list of callbacks is also available globally and may be updated by
other threads so we need to make sure we use the right lock.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16980)