[FIX] crm: prevent division by zero in lead probability computation

If for some reason we cannot compute lead probability, we should simply
continue to the next value without causing a division by zero.

This commit is a backport of part of https://github.com/odoo/odoo/pull/76110

.

Task-2674586

closes odoo/odoo#78765

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>