is inconsistent with the MySQL .frm file.

ha_innobase::index_read(): When the index cannot be found,
return an error.

ha_innobase::change_active_index(): When prebuilt->index == NULL,
set also prebuilt->index_usable = FALSE.  This is not needed for
correctness, because prebuilt->index_usable is only checked by
row_search_for_mysql(), which requires prebuilt->index != NULL.

This addresses Issue #349.  Approved by Heikki Tuuri over IM.