Don't use an uninitialised variable in srp application

The srp application created an uninitialised DB_ATTR object and then
passed it to the load_index function which attempted to read it. A
DB_ATTR object only contains a single field called "unique_subject".
AFAICT this attribute is unused in the SRP case, and therefore it would be
better to pass a NULL DB_ATTR to load_index (which handles that case
gracefully).

Reviewed-by: Rich Salz <rsalz@openssl.org>