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()).

(cherry picked from commit a531676d6ddcda78f126781ca628cc2d5fc20277)
(cherry picked from commit f511a2915ed856f048fd7dd91fd9f7537b1e0479)

Change-Id: I88b563b88beebcd1c3cb21fe61bb56cdb12cdc41
Reviewed-on: https://gerrit.libreoffice.org/9882
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>