sw: tdf#119742 fix null pointer in SetCursorInHdFt

In SwWrtShell::ChangeHeaderOrFooter the ChgPageDesc destroys the frames
of the header, that's why GetCurrFrame returns null later.

Change-Id: I26915237c8b455a5b8ad0bbd5c2de38537dfef08
Reviewed-on: https://gerrit.libreoffice.org/60130Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>
Tested-by: Michael Stahl <Michael.Stahl@cib.de>

sw/source/core/crsr/crstrvl.cxx

@@ -199,7 +199,8 @@ bool SwCursorShell::SetCursorInHdFt( size_t nDescNo, bool bInHeader )
     if( SIZE_MAX == nDescNo )
     {
         // take the current one
-        const SwPageFrame* pPage = GetCurrFrame()->FindPageFrame();
+        const SwContentFrame *pCurrFrame = GetCurrFrame();
+        const SwPageFrame* pPage = (pCurrFrame == nullptr) ? nullptr : pCurrFrame->FindPageFrame();
         if( pPage && pMyDoc->ContainsPageDesc(
                 pPage->GetPageDesc(), &nDescNo) )
             pDesc = pPage->GetPageDesc();