rhbz#1075124: writerfilter: fix tables with negative left margin

SvxBoxItem::nLeftDist is unsigned; the bugdoc sets a very large value
due to wrap-around which causes the table content not to be displayed.

DomainMapperTableHandler::endTableGetTableStyle() tweaks the
rInfo.nLeftBorderDistance by adding half the border line width to it,
which makes it positive; at that point the value has already been used
to init table::TableBorderDistances, so move that downward so it gets
the positive value too.

Change-Id: Ied2331b93f2e95845d7e8b2cc06e89e1ab24296e