Clear() an SdPage while it is still an SdPage

... because when Clear() is later called in the SdrObjList sub-object dtor,
freed SdrObject dtors may call back via SdrObject::SendUserCall into this
virtual SdPage::Changed, but then this is only an SdrObjList, causing undef
behavior.

Change-Id: I09a2d1445a51a70e8f795ff869027899ad0f528b