Since CWS swlayoutrefactoring the SwRootFrms are destroyed with
SwDoc::IsInDtor not set.  This can cause at least reads of freed
SwRootFrm members when executing the dtors of SwRootFrm base class
SwLayoutFrm calling into SwRootFrm::GetPageAtPos.

Prevent this scenario by:
- moving the implementation of SwRootFrm base class dtors to new
  methods SwFrm::Destroy and SwLayoutFrm::Destroy
- calling SwFrm::Destroy and SwLayoutFrm::Destroy explicitly before
  SwRootFrm members are freed