[1.2.X] Fixed #15075 - No longer possible to alter the form_list in FormWizard.process_step

Thanks to niels, stas for the report, and stas for the patch.

Backport of [15196] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15197 bcc190cf-cafb-0310-a4f2-bffc1f526a37