[FIX] mass_mailing: correctly find partner email in mass mailing

In mass mailing access to partners when performing a mass mailing has been
done in batch to speedup computation [1]. Emails are put into a dictionary
allowing to find back the email based on partner_id.

However the matching between the partner and its emails is done using a
shortcut using the current document ID as partner ID. It works when performing
a mass mailing on partners but fails when performing a mass mailing on models
having message_get_default_recipients not returning only emails. Currently
in saas-14 main models return only emails (crm, event, mailing contacts) but
other models may encounter issues (applicants, tickets).

This commit fixes it by correctly matching partner id and its found email.

[1] See 65ed4553a5

addons/mass_mailing/wizard/mail_compose_message.py

@@ -62,7 +62,8 @@ class MailComposeMessage(models.TransientModel):
                 if mail_values.get('email_to'):
                     recips = tools.email_split(mail_values['email_to'])
                 else:
-                    recips = tools.email_split(partners_email.get(res_id))
+                    partner_id = (mail_values.get('recipient_ids') or [(False, '')])[0][1]
+                    recips = tools.email_split(partners_email.get(partner_id))
                 mail_to = recips[0].lower() if recips else False
                 if (blacklist and mail_to in blacklist) or (seen_list and mail_to in seen_list):
                     # prevent sending to blocked addresses that were included by mistake