Recreating a user fails with NPE

[marcelmay]

Reported in #521 for version 1.6.12

Re-creating a user fails with NPE when receiving mails

802     ERROR      commands.DataCommand| Can not send state '250 OK', aborted.
java.lang.IllegalStateException: Can not deliver message MovingMessage{toAddresses=[foo@localhost], returnPath=foo@localhost, message=jakarta.mail.internet.MimeMessage@7ebdad14} to foo@localhost
	at com.icegreen.greenmail.smtp.SmtpManager$Incoming.handle(SmtpManager.java:89) ~[classes/:?]
	at com.icegreen.greenmail.smtp.SmtpManager$Incoming.enqueue(SmtpManager.java:80) ~[classes/:?]
	at com.icegreen.greenmail.smtp.SmtpManager.send(SmtpManager.java:55) ~[classes/:?]
	at com.icegreen.greenmail.smtp.commands.DataCommand.execute(DataCommand.java:63) ~[classes/:?]
	at com.icegreen.greenmail.smtp.SmtpHandler.handleCommand(SmtpHandler.java:92) ~[classes/:?]
	at com.icegreen.greenmail.smtp.SmtpHandler.run(SmtpHandler.java:46) ~[classes/:?]
	at com.icegreen.greenmail.server.AbstractServer.lambda$handleClientSocket$0(AbstractServer.java:165) ~[classes/:?]
	at java.lang.Thread.run(Thread.java:750) ~[?:1.8.0_362]
Caused by: java.lang.IllegalStateException: Can not deliver MovingMessage{toAddresses=[foo@localhost], returnPath=foo@localhost, message=jakarta.mail.internet.MimeMessage@7ebdad14} for user UserImpl{email='foo@localhost', login='foo@localhost'}
	at com.icegreen.greenmail.user.UserImpl.deliver(UserImpl.java:65) ~[classes/:?]
	at com.icegreen.greenmail.user.UserManager.deliver(UserManager.java:146) ~[classes/:?]
	at com.icegreen.greenmail.smtp.SmtpManager$Incoming.handle(SmtpManager.java:87) ~[classes/:?]
	... 7 more
Caused by: java.lang.NullPointerException
	at com.icegreen.greenmail.user.UserImpl.deliver(UserImpl.java:63) ~[classes/:?]
	at com.icegreen.greenmail.user.UserManager.deliver(UserManager.java:146) ~[classes/:?]
	at com.icegreen.greenmail.smtp.SmtpManager$Incoming.handle(SmtpManager.java:87) ~[classes/:?]
	... 7 more

java.lang.IllegalStateException: Can not send message jakarta.mail.internet.MimeMessage@72967906

	at com.icegreen.greenmail.util.GreenMailUtil.sendMimeMessage(GreenMailUtil.java:272)
	at com.icegreen.greenmail.util.GreenMailUtil.sendTextEmail(GreenMailUtil.java:260)
	at com.icegreen.greenmail.test.SmtpServerTest.testSendAndReCreateUser(SmtpServerTest.java:254)
...
Caused by: com.sun.mail.smtp.SMTPSendFailedException: 451 Requested action aborted: local error in processing
	at com.sun.mail.smtp.SMTPTransport.issueSendCommand(SMTPTransport.java:2374)
	at com.sun.mail.smtp.SMTPTransport.finishData(SMTPTransport.java:2095)
	at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:1301)
	at jakarta.mail.Transport.send0(Transport.java:231)
	at jakarta.mail.Transport.send(Transport.java:100)
	at com.icegreen.greenmail.util.GreenMailUtil.sendMimeMessage(GreenMailUtil.java:270)
	... 30 more