shared mailboxes: Don't crash if trying to open "shared/domain" namespace prefix as mailbox.
shared mailboxes: Don't crash if trying to open "shared/domain" namespace prefix as mailbox.

--HG--
branch : HEAD

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