lib-index: Fix detecting if index is undeleted

lib-index: Fix detecting if index is undeleted

It was possible that mail_index_is_deleted() kept returning that the index
was deleted, even if it was marked as undeleted. This happened when the
deletion was in dovecot.index.log.2, undeletion in dovecot.index.log and
dovecot.index pointed to the .log.2 file. This caused the .log file to be
read first, which sees the undeletion. Later on the .log.2 was parsed and
it marked the index back as deleted.