apparently some table .docx properties shouldn't be < 0

Somewhat related to 10b4da63e3143108ba75891e9e98fdaa2f7953ab , a similar
doc has negative value inside w:tblCellMar, which MSO seems to ignore
(altering the value has no visible effect), so ignore it as well.

Change-Id: I846e9b55fea0d4e66f03ce615584516360b8b7dd
Reviewed-on: https://gerrit.libreoffice.org/8764
Reviewed-by: Tor Lillqvist <tml@collabora.com>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>