Regression from commit a5a836d8 (DOCX
filter: effect extent should be part of the margin, 2014-12-04), the
effect extent is added to the nominal margin in DOCX, so we exclude that
from the margin in our document model. But it shouldn't be ever
negative, ST_WrapDistance is a restriction of the W3C XML Schema
unsignedInt datatype.

Change-Id: I82b3c1ba0e3a14f7c585b0d389264a2c12e454e7
Reviewed-on: https://gerrit.libreoffice.org/31064Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>