crypto: use SSL_get_SSL_CTX.

SSL_get_SSL_CTX returns the SSL_CTX for an SSL. Previously the code
accessed |ssl->ctx| directly, but that's no longer possible with OpenSSL
1.1.0.

SSL_get_SSL_CTX exists all the way back to (at least) OpenSSL 0.9.8 and
so this change should be fully compatible.

PR-URL: https://github.com/nodejs/node/pull/8995


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Shigeki Ohtsu <ohtsu@ohtsu.org>