XCLIENT client passthrough has an empty callback

[xPaw]

https://github.com/nodemailer/smtp-server/blob/93e42a1b2aeab94cd4cd9db87eb874b4f807385c/lib/smtp-server.js#L198-L201

This causes a crash here:

TypeError: Cannot read property 'user' of undefined
    at node_modules/smtp-server/lib/sasl.js:461:31
    at SMTPServer.onAuth (node_modules/smtp-server/lib/smtp-server.js:199:20)
    at SMTPConnection.SASL_XCLIENT (node_modules/smtp-server/lib/sasl.js:436:22)
    at checkLogin (node_modules/smtp-server/lib/smtp-connection.js:931:26)
    at SMTPConnection.handler_XCLIENT (node_modules/smtp-server/lib/smtp-connection.js:950:9)
    at SMTPConnection._onCommand (node_modules/smtp-server/lib/smtp-connection.js:476:17)
    at SMTPStream.SMTPConnection._parser.oncommand (node_modules/smtp-server/lib/smtp-connection.js:58:52)
    at readLine (node_modules/smtp-server/lib/smtp-stream.js:127:22)
    at SMTPStream._write (node_modules/smtp-server/lib/smtp-stream.js:131:13)