Fix dead lock when showDotAndDotDot is set to FolderListModel

Task-number: QTBUG-31687

Change-Id: Iaec798018f54f80c67fe5a06defee73ab7b79605
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>