mbox: Don't return mailbox GUID if it's coming from in-memory index.
mbox: Don't return mailbox GUID if it's coming from in-memory index.

--HG--
branch : HEAD

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