set only valid positions from ScExtTabSettings at ScViewDataTable

Some address fields of ScExtTabSettings are initialized invalid and
maybe kept invalid, on purpose, so take care of that and use only the
valid fields.

Change-Id: I37cbf8ff49c6ccf38bfac3e1c8c6492ac7bad61b
(cherry picked from commit 8bc56801af0540c0496c1f8ddd335578a8791017)
Reviewed-on: https://gerrit.libreoffice.org/13182
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>