tls: do not crash on STARTTLS when OCSP requested

`TLSSocket` should not have a hard dependency on `tls.Server`, since it
may be running without it in cases like `STARTTLS`.

Fix: #10704
PR-URL: https://github.com/nodejs/node/pull/10706


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>