[FIX] digest: fix m2m typo that would not do whats expected

m2m ORM operation (4, id, _) was not correctly used, resulting in an
unexpected behavior. Instead of adding the record, it would add the record and
also the res.user(4,).

As that used is generally the public user, without an email address, that bug
would not be that critical but if that res.user(4,) does not exist (eg deleted),
then the code simply crash as it tries to read user 4 from db.

closes odoo/odoo#32425

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>

addons/digest/models/digest.py

@@ -133,7 +133,7 @@ class Digest(models.Model):
         tip = self.env['digest.tip'].search([('user_ids', '!=', user.id), '|', ('group_id', 'in', user.groups_id.ids), ('group_id', '=', False)], limit=1)
         if not tip:
             return False
-        tip.user_ids = [4, user.id]
+        tip.user_ids += user
         body = tools.html_sanitize(tip.tip_description)
         tip_description = self.env['mail.template']._render_template(body, 'digest.tip', self.id)
         return tip_description