ldb_tdb: consistently use ltdb->cache->attribute_indexes to determine if we have...

ldb_tdb: consistently use ltdb->cache->attribute_indexes to determine if we have indexes

This is instead of checking the number of elements via ltdb->cache->indexlist->num_elements

In turn, this allows us to avoid fetching ltdb->cache->indexlist in the future

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>