Don't use uninitialized ImplSplitItem::mnPixSize

...as happened e.g. during CppunitTest_sw_ooxmlexport4 when

 SplitWindow::GetItemSize
 SfxSplitWindow::InsertWindow_Impl
 SfxSplitWindow::InsertWindow
 SfxDockingWindow::Initialize
 sfx2::sidebar::SidebarChildWindow::SidebarChildWindow
 sfx2::sidebar::SidebarChildWindow::CreateImpl
 SfxChildWindow::CreateChildWindow
 SfxWorkWindow::CreateChildWin_Impl
 SfxWorkWindow::UpdateChildWindows_Impl
 SfxWorkWindow::UpdateObjectBars_Impl
 SfxFrameWorkWin_Impl::UpdateObjectBars_Impl
 SfxDispatcher::Update_Impl
 SfxBaseController::ConnectSfxFrame_Impl
 SfxBaseController::attachFrame
 ...

requests the SWIB_FIXED of an SWIB_PERCENTSIZE item.  Hopefully zero is fine
here and the value is only used for ephemeral computations that are overriden
later with true sizes?

Change-Id: I250b8fa38e42c111eef0bcedd703181e548a6b60

vcl/source/window/splitwin.cxx

@@ -2677,6 +2677,7 @@ void SplitWindow::InsertItem( sal_uInt16 nId, vcl::Window* pWindow, long nSize,
     // Create new item
     ImplSplitItem* pItem = new ImplSplitItem();
     pItem->mnSize   = nSize;
+    pItem->mnPixSize = 0;
     pItem->mnId     = nId;
     pItem->mnBits   = nBits;
     pItem->mnMinSize=-1;