[FIX] base: multi-pages and multi-records traceback

In case of a multi-record and multi-page (i.e. 2 pages per PDF) report
the assertion is triggered b/c we have 4 pages and only two records with
a PDF of 2 pages each, so the outline_page needs to be unique before
assertion.

len([0, 0, 2, 2]) != len([record_id1, record_id2])
len([0, 2]) == len([record_id1, record_id2])

closes odoo/odoo#39701

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>