[IMP] l10n_ar: extend constraint on pos number to ensure max 5 digits

Before this change we were letting the user to create a journal with a
AFIP POS Number they like, and where they try to post an invoice in that
jounrals they receive an error message telling that the document has not
valid POS Number.

In order to avoid this we add as part of the constraint where we check
the validity of the AFIP POS Number which is: should be greater that 0,
and should be 5 digits max.

closes odoo/odoo#43879

Signed-off-by: Josse Colpaert <jco@openerp.com>