dsync: Avoid assert-crashing when trying to create a reserved mailbox name (e.g. dbox-Mails)
dsync: Avoid assert-crashing when trying to create a reserved mailbox name (e.g. dbox-Mails)

jwm/dovecot-notmuch,jkerihuel/dovecot,jkerihuel/dovecot,jkerihuel/dovecot,jkerihuel/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch,jkerihuel/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch