mailbox_list_mailbox(): Avoid crashing with shared namespace prefix.
mailbox_list_mailbox(): Avoid crashing with shared namespace prefix.

--HG--
branch : HEAD

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