Sync causes Master Password prompt in infinite loop

See: 
- https://groups.google.com/d/topic/greasemonkey-users/StRnQOdU7-4/discussion
- https://github.com/greasemonkey/greasemonkey/issues/1573#issuecomment-31245611
