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
diff --git a/sd/source/core/sdpage.cxx b/sd/source/core/sdpage.cxx
index 8f8f6e6..9e368eb 100644
--- a/sd/source/core/sdpage.cxx
+++ b/sd/source/core/sdpage.cxx
@@ -147,6 +147,8 @@ SdPage::~SdPage()

    if( mpItems )
        delete mpItems;

    Clear();
}

struct OrdNumSorter