lib-storage: Don't crash if INBOX is in a namespace with non-"" non-"INBOX/" prefix.
lib-storage: Don't crash if INBOX is in a namespace with non-"" non-"INBOX/" prefix.

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