Fix a crash with multi-threaded applications using the FIPS module

Fix a crash with multi-threaded applications using the FIPS module

The FIPS implementation of the ossl_ctx_thread_stop function needs to
use an OSSL_LIB_CTX - but gets passed a provctx as an argument. It was
assuming that these are the same thing (which was true at one point
during development) - but that is no longer the case. The fix is to
get the OSSL_LIB_CTX out of the provctx.

Fixes #13469

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/13660)