coverity#1436014 silence Dereference null return value

Change-Id: I5ba1f864b5fcf69e3437792f99d24367f5f5a44d
Reviewed-on: https://gerrit.libreoffice.org/55603Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sw/source/core/undo/rolbck.cxx

@@ -490,6 +490,7 @@ void SwHistorySetFootnote::SetInDoc( SwDoc* pDoc, bool )
         SwTextFootnote * const pFootnote =
             static_cast<SwTextFootnote*>(
                 pTextNd->GetTextAttrForCharAt( m_nStart ));
+        assert(pFootnote);
         SwFormatFootnote &rFootnote = const_cast<SwFormatFootnote&>(pFootnote->GetFootnote());
         rFootnote.SetNumStr( m_FootnoteNumber  );
         if ( rFootnote.IsEndNote() != m_bEndNote )