Update class.form.php

I encountered the issue described here: http://vanillaforums.org/discussion/19548/saving-settings-in-dashboard-throws-error-permalink-issues-vanilla-v2-1a, but on version 2.0.18.5. I tried the fix mentioned in http://vanillaforums.org/discussion/comment/156870/#Comment_156870, and it fixed the issue.