fix broken redlines after undoing column count change (cp#1000071)

Changing a document with redlines to two columns and then using undo
set some redlines to point to the end of the document (done by
the call to RemoveIdxFromSection() in SwUndoInsSection::UndoImpl()).

Change-Id: I88b563b88beebcd1c3cb21fe61bb56cdb12cdc41