crypto/init.c: use destructor_key even as guard in OPENSSL_thread_stop.

Problem was that Windows threads that were terminating before libcrypto
was initialized were referencing uninitialized or possibly even
unrelated thread local storage index.

Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/6752)