Fix crash by ignoring data after SMTPClient has been destroyed 

This pull request fixes a crash I encountered.

When sending an e-mail to a recipient with a weird e-mail address (say, containing a % or a !), my mail server (Exim4) will reject it.

This makes simplesmtp in lib/client.js:783 raise an error:

        this._onError(new Error("Can't send mail - no recipients defined"), "RecipientError");

The _onError handler closes the connection by calling this.close() which in turn calls _destroy.

The culprit is, with my local mail server (it doesn't happen with a remote server on another machine), _onData is called once more *after* _destroy has been called. This leads to a unhandled error event (and application crash) since the 'error' listener was cleared by _destroy already:

    events.js:68
            throw arguments[1]; // Unhandled 'error' event
                           ^
    RecipientError: Can't send mail - all recipients were rejected
        at SMTPClient._actionRCPT ([...]/node_modules/nodemailer/node_modules/simplesmtp/lib/client.js:808:27)
        at SMTPClient._onData ([...]/node_modules/nodemailer/node_modules/simplesmtp/lib/client.js:261:29)
        at Socket.EventEmitter.emit (events.js:93:17)
        at TCP.onread (net.js:396:14)

Setting _ignoreData to true in _destroy avoids processing any data after listeners have been cleared.