guard against missing SuperTable

Change-Id: Ic7cc6c807905e0c4ffbf2a3f009b27be6100cdf0
(cherry picked from commit 0cde3ca2)
Reviewed-on: https://gerrit.libreoffice.org/20540Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

lotuswordpro/source/filter/lwpfribtable.cxx

@@ -77,7 +77,9 @@ LwpSuperTableLayout* LwpFribTable::GetSuperTable()
 
 void LwpFribTable::RegisterNewStyle()
 {
-    GetSuperTable()->RegisterNewStyle();
+    LwpSuperTableLayout* pSuper = GetSuperTable();
+    if (pSuper)
+        pSuper->RegisterNewStyle();
     XFParaStyle* pOldStyle = m_pPara->GetXFParaStyle();
     if(HasNextFrib())
     {