[FIX] mail: do not crash if a partner has more than one mail.channel.partner
[FIX] mail: do not crash if a partner has more than one mail.channel.partner

It is assumed that there is a unique mail.channel.partner per (channel, partner)
pair, yet there is nothing enforcing this restriction.
As a result field access on the channel partner crashes on field access.

closes odoo/odoo#31259
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo