sw: check 0 paint window in RenderContextGuard

As seen by Norbert during CppunitTest_sw_globalfilter.

Change-Id: I02dfab78b3cdcc65187f04e86cce7b7432c0a138
Reviewed-on: https://gerrit.libreoffice.org/17677Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>

sw/source/core/view/viewsh.cxx

@@ -1677,8 +1677,8 @@ public:
             rtl::Reference<sdr::overlay::OverlayManager> xOldManager = pOldPaintWindow->GetOverlayManager();
             if (xOldManager.is())
             {
-                SdrPaintWindow* pNewPaintWindow = m_pShell->Imp()->GetDrawView()->FindPaintWindow(*m_pRef);
-                xOldManager->completeRedraw(pNewPaintWindow->GetRedrawRegion(), m_pRef);
+                if (SdrPaintWindow* pNewPaintWindow = m_pShell->Imp()->GetDrawView()->FindPaintWindow(*m_pRef))
+                    xOldManager->completeRedraw(pNewPaintWindow->GetRedrawRegion(), m_pRef);
             }
 
             m_pShell->Imp()->GetDrawView()->DeleteWindowFromPaintView(m_pRef);