tls: fix tls handshake check in ssl error

In ssl.onerror event, `this` refers `ssl` so that
`this._secureEstablished` is always undefined. Fix it to refer
TLSSocket.

PR-URL: https://github.com/nodejs/io.js/pull/1661


Reviewed-By: Fedor Indutny <fedor@indutny.com>