guard against no default text style

Change-Id: I7416d709de79ae88cf4a9546d8adfecec9935238
(cherry picked from commit 80297dd0)
Reviewed-on: https://gerrit.libreoffice.org/20528Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

lotuswordpro/source/filter/lwptoc.cxx

@@ -125,7 +125,8 @@ void LwpTocSuperLayout::RegisterStyle()
     LwpSuperTableLayout::RegisterStyle();
 
     // Get font info of default text style and set into tab style
-    XFParaStyle* pBaseStyle = static_cast<XFParaStyle*>(m_pFoundry->GetStyleManager()->GetStyle(*m_pFoundry->GetDefaultTextStyle()));
+    const LwpObjectID *pDefaultTextStyle = m_pFoundry->GetDefaultTextStyle();
+    XFParaStyle* pBaseStyle = pDefaultTextStyle ? static_cast<XFParaStyle*>(m_pFoundry->GetStyleManager()->GetStyle(*pDefaultTextStyle)) : nullptr;
     XFTextStyle*pTextStyle = new XFTextStyle;
     if (pBaseStyle)
         pTextStyle->SetFont(pBaseStyle->GetFont()); // who delete this font?????