[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.

addons/mail/models/mail_channel.py

@@ -730,7 +730,7 @@ def channel_seen(self):
         self.ensure_one()
         if self.channel_message_ids.ids:
             last_message_id = self.channel_message_ids.ids[0] # zero is the index of the last message
-            channel_partner = self.env['mail.channel.partner'].search([('channel_id', 'in', self.ids), ('partner_id', '=', self.env.user.partner_id.id)])
+            channel_partner = self.env['mail.channel.partner'].search([('channel_id', 'in', self.ids), ('partner_id', '=', self.env.user.partner_id.id)], limit=1)
             if channel_partner.seen_message_id.id == last_message_id:
                 # last message seen by user is already up-to-date
                 return
@@ -761,7 +761,7 @@ def channel_fetched(self):
             if channel.channel_type != 'chat':
                 return
             last_message_id = channel.channel_message_ids.ids[0] # zero is the index of the last message
-            channel_partner = self.env['mail.channel.partner'].search([('channel_id', '=', channel.id), ('partner_id', '=', self.env.user.partner_id.id)])
+            channel_partner = self.env['mail.channel.partner'].search([('channel_id', '=', channel.id), ('partner_id', '=', self.env.user.partner_id.id)], limit=1)
             if channel_partner.fetched_message_id.id == last_message_id:
                 # last message fetched by user is already up-to-date
                 return