This RES_FRM_SIZE item is apparently SwFormatFrameSize, not SvxSizeItem

...as reported by -fsanitize=vptr when doing "Format - Page..." in Writer
(though both types have a Size member at the same location, so didn't cause any
real problems):

> sw/source/ui/misc/pgfnote.cxx:283:32: runtime error: downcast of address 0x604000e23f50 which does not point to an object of type 'const SvxSizeItem'
> 0x604000e23f50: note: object is of type 'SwFormatFrameSize'
>  94 01 00 3d  90 ae ee 90 ff 7e 00 00  06 00 00 00 58 00 00 be  d0 2f 00 00 00 00 00 00  e0 3d 00 00
>               ^~~~~~~~~~~~~~~~~~~~~~~
>               vptr for 'SwFormatFrameSize'
>     #0 0x7efd1c8d01e2 in SwFootNotePage::ActivatePage(SfxItemSet const&) sw/source/ui/misc/pgfnote.cxx:283:32
>     #1 0x7efd1c8cbf07 in SwFootNotePage::Reset(SfxItemSet const*) sw/source/ui/misc/pgfnote.cxx:230:5
>     #2 0x7f002e149560 in SfxTabDialog::ActivatePageHdl(TabControl*) sfx2/source/dialog/tabdlg.cxx:1117:19
>     #3 0x7f002e1400e3 in SfxTabDialog::LinkStubActivatePageHdl(void*, TabControl*) sfx2/source/dialog/tabdlg.cxx:1035:1
>     #4 0x7f0008248f37 in Link<TabControl*, void>::Call(TabControl*) const include/tools/link.hxx:84:45
>     #5 0x7f0008204caa in TabControl::ActivatePage() vcl/source/control/tabctrl.cxx:1601:19

Change-Id: I73df2438565a7069153b22140197897df810b2aa
diff --git a/sw/source/ui/misc/pgfnote.cxx b/sw/source/ui/misc/pgfnote.cxx
index 691105f..44f6d03 100644
--- a/sw/source/ui/misc/pgfnote.cxx
+++ b/sw/source/ui/misc/pgfnote.cxx
@@ -18,6 +18,7 @@
 */

#include <cmdid.h>
#include <fmtfsize.hxx>
#include <hintids.hxx>
#include <svx/dialmgr.hxx>
#include <svx/dialogs.hrc>
@@ -280,8 +281,8 @@ bool SwFootNotePage::FillItemSet(SfxItemSet *rSet)

void SwFootNotePage::ActivatePage(const SfxItemSet& rSet)
{
    const SvxSizeItem& rSize = static_cast<const SvxSizeItem&>(rSet.Get( RES_FRM_SIZE ));
    lMaxHeight = rSize.GetSize().Height();
    auto const & rSize = static_cast<SwFormatFrameSize const &>(rSet.Get( RES_FRM_SIZE ));
    lMaxHeight = rSize.GetHeight();

    const SfxPoolItem* pItem;
    if( SfxItemState::SET == rSet.GetItemState( rSet.GetPool()->GetWhich( SID_ATTR_PAGE_HEADERSET), false, &pItem ) )