tls_wrap: Unlink TLSWrap and SecureContext objects

This makes `TLSWrap` and `SecureContext` objects collectable by the
incremental gc.

`res = null` destroys the cyclic reference in the `reading` property.
`this.ssl = null` removes the remaining reference to the `TLSWrap`.
`this.ssl._secureContext.context = null` removes the reference to
the `SecureContext` object, even though there might be references
to `this.ssl._secureContext` somewhere.

The `reading` property will now throw an error if accessed after the
socket is closed, but that should not happen.

PR-URL: #1580
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>

lib/_tls_wrap.js

@@ -289,15 +289,22 @@ TLSSocket.prototype._wrapHandle = function(handle) {
  }
  });
 
- this.on('close', this._destroySSL);
+ this.on('close', function() {
+ this._destroySSL();
+ res = null;
+ });
 
  return res;
 };
 
 TLSSocket.prototype._destroySSL = function _destroySSL() {
+ if (!this.ssl) return;
  this.ssl.destroySSL();
- if (this.ssl._secureContext.singleUse)
+ if (this.ssl._secureContext.singleUse) {
  this.ssl._secureContext.context.close();
+ this.ssl._secureContext.context = null;
+ }
+ this.ssl = null;
 };
 
 TLSSocket.prototype._init = function(socket, wrap) {