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) {