QmlDesigner.FormEditor: crash fix

Anchors and/or anchor targets might be invalid.
e.g.: anchors.left: parent

We have to check this.

Task-number: QTCREATORBUG-11255
Change-Id: Ib939292d0f89cc1d22119972627c03f1e85043db
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>