[1.2.X] Fixed #14498 - Forms passed to FormWizard.process_step are not...

[1.2.X] Fixed #14498 - Forms passed to FormWizard.process_step are not guaranteed to have cleaned_data

Thanks to stas for the report.

Backport of [14290] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14291 bcc190cf-cafb-0310-a4f2-bffc1f526a37