{FIX] website_event_sale: Updating cart with unavailable ticket

Steps to reproduce the bug:
- Create an event E with max available seats = 1
- Register to E with public user and leave the cart
- In backend, register an attendee for E and confirm it
- Go to the cart, try to update the line with the registration

Bug:

The line was not updated to 0 and so the customer had to buy an unavailable
ticket.

opw:2091720

closes odoo/odoo#39338

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>