since...

commit a4cd8415
Date:   Thu Feb 13 16:10:47 2014 +0530

    n#757432: Styles (rename &) copy to different decks.

    While copying slides to different slide decks,
    styles were not being copied if there is already one
    with the same name. This patch renames and copies those
    to keep the formatting intact.

if a SdStyleSheet is not freshly created, but reused, it ends up in
the rCreatedSheets list, and so when the copy is undone
(via SdMoveStyleSheetsUndoAction), the SdStyleSheet is unconditionally
remove from the StylePool and so its ref count hits 0 and is removed,
even though there are still objects depending on it.

the problem becames more common since...

bash-4.4$ git show 57db6e24
commit 57db6e24
Date:   Tue Aug 15 02:51:07 2017 +0530

    Removing old SfxItemSet::getHash usage

improves the SfxItemSet comparison.

Assuming that its correct to leave SdStyleSheetPool::CopySheets
with those reused SdStyleSheets in the list, change things so
that a flag is passed around to indicate if the SdStyleSheet
should be removed or not by undo

Change-Id: I82b5bd93183fd1ff9e67957ccfb5babef81fd36d
Reviewed-on: https://gerrit.libreoffice.org/59432
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>