(stable) Fix an EditEngine issue with unlocking fields which can't be locked

Summary: This code should go inside the field-locking loop. Otherwise, it only applies to the last field, and fatals if there are no fields.

Test Plan: Carefuller inspection.

Reviewers: chad

Reviewed By: chad

Subscribers: 20after4

Differential Revision: https://secure.phabricator.com/D16889