[FIX] website_sale: multi-website

The fix added in 3a0f05f3 doesn't work if an already existing user in company A
trys to modify his address on a website linked to company B. The form will try to change the company_id
of the res.partner which will then conflit with the company_id of the associated res.user, ultimately leading
to a UserError and a 500 error on the website.

opw-2025457

closes odoo/odoo#34262

Signed-off-by: Rémi Rahir (rar) <rar@odoo.com>