tdf#96745: Hold VclPtr on a border window's client window

It was Noel's idea ... makes sense given that following Close()
call not only closes, but also deletes the window

Change-Id: Icea766d2a7bb7bd834f32080f7c69a9fc67ebf54
Reviewed-on: https://gerrit.libreoffice.org/21878Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/source/window/brdwin.cxx

@@ -960,9 +960,9 @@ bool ImplStdBorderWindowView::Tracking( const TrackingEvent& rTEvt )
                 {
                     // dispatch to correct window type (why is Close() not virtual ??? )
                     // TODO: make Close() virtual
-                    vcl::Window *pWin = pBorderWindow->ImplGetClientWindow()->ImplGetWindow();
-                    SystemWindow  *pSysWin  = dynamic_cast<SystemWindow* >(pWin);
-                    DockingWindow *pDockWin = dynamic_cast<DockingWindow*>(pWin);
+                    VclPtr<vcl::Window> pWin = pBorderWindow->ImplGetClientWindow()->ImplGetWindow();
+                    SystemWindow  *pSysWin  = dynamic_cast<SystemWindow* >(pWin.get());
+                    DockingWindow *pDockWin = dynamic_cast<DockingWindow*>(pWin.get());
                     if ( pSysWin )
                         pSysWin->Close();
                     else if ( pDockWin )