[REVIEW][3-5][3-4] more problems in SwRootFrm dtor

[Michael Stahl]

due to CWS swlayoutrefactoring there is another problem in SwRootFrm
destruction, which has been found with valgrind:
the children of the root frame are only destroyed by the base class
destructors, so since every layout frame has a pointer to the SwRootFrm
they may access its members after the SwRootFrm dtor has returned
(valgrind complained about an invalid read of a freed SwRootFrm member).

so refactor this a little to execute the base class dtors before
SwRootFrm dtor returns:

http://cgit.freedesktop.org/libreoffice/core/commit/?id=ebb74441790a9852b1a1532d6e025c324666f6fc

plus, valgrind also complained about this:

http://cgit.freedesktop.org/libreoffice/core/commit/?id=f430b71c46186b53a6364a60b237c81245f3eabb