XCLIENT client passthrough has an empty callback

[xPaw]

smtp-server/lib/smtp-server.js

Lines 198 to 201 in 93e42a1

	if (auth.method === 'XCLIENT') {
	return callback(); // pass through
	}

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)