tls: nullify `.ssl` on handle close

This is an intermediate fix for an issue of accessing `TLSWrap` fields
after the parent handle was destroyed. While `close` listener cleans up
this field automatically, it can be done even earlier at the
`TLSWrap.close` call.

Proper fix is going to be submitted and landed after this one.

Fix: #5108
PR-URL: https://github.com/nodejs/node/pull/5168


Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>