crashtesting: crash on import of ooo33887.sxw

since

commit a172f854b6
Date:   Sun Dec 3 21:00:37 2017 +0100

    tdf#74693: Footnotes text appearing above footnote separator line

    When invalidate a footnoteframe, invalidate also the lower textframe,
    so it will be recalculated before text rendering.

Change-Id: I95aec5e86bc53126de71bdebe7fbfada955be9c9
Reviewed-on: https://gerrit.libreoffice.org/46085
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sw/source/core/layout/frmtool.cxx

@@ -547,8 +547,11 @@ SwLayNotify::~SwLayNotify()
     if ( bPos && pLay->Lower() && !IsLowersComplete() )
     {
         pLay->Lower()->InvalidatePos();
-        if(pLay->Lower()->IsFootnoteFrame())
-            static_cast<SwFootnoteFrame*>(pLay->Lower())->Lower()->InvalidatePos();
+        SwFootnoteFrame* pFtnFrame = pLay->Lower()->IsFootnoteFrame() ?
+            static_cast<SwFootnoteFrame*>(pLay->Lower()) : nullptr;
+        SwFrame* pFtnLower = pFtnFrame ? pFtnFrame->Lower() : nullptr;
+        if (pFtnLower)
+            pFtnLower->InvalidatePos();
     }
 
     if ( bPrtPos )