mailbox_list_index: Fixed assert-crash sometimes when deleting a mailbox.
mailbox_list_index: Fixed assert-crash sometimes when deleting a mailbox.

LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot