JIRA does not handle usernames with line breaks correctly

JIRA allows users to be created with line breaks through the JIRA Importers Plugin (and possibly other plugins), but does not handle these users correctly.

Steps to replicate:

1. Import a user, containing a line break in the username, through the CSV importer (JIM-1001);
2. Try to edit or delete these users.

Expected Behavior:

The administrator would be able to edit or delete the broken users.

Actual Behavior:

It is not possible to edit or delete this user. The following appears on the logs:

2013-07-01 17:20:32,465 http-bio-8090-exec-4 ERROR      [500ErrorPage.jsp] Exception caught in 500 page null
java.lang.NullPointerException
	at com.atlassian.jira.web.action.admin.user.EditUser.buildNewUser(EditUser.java:106)
	at com.atlassian.jira.web.action.admin.user.EditUser.doValidation(EditUser.java:57)
	at webwork.action.ActionSupport.validate(ActionSupport.java:391)
...

Workaround:

1. Create a backup of JIRA;
2. Unzip the backup;
3. Open the XML backups in a text editor;
4. Use the 'Find and Replace' feature of the text editor to replace e.g. 'Matheus
   Fernandes' with 'matheusfernandes';
5. Save the files, zip them together again;
6. And re-import the backup.
   ^{Reference: 'User does not exist' error when trying to use, edit or delete an user}