Let ipsp_spd_lookup() return an error instead of a TDB. The TDB

Let ipsp_spd_lookup() return an error instead of a TDB.  The TDB
is not always needed, but the error value is necessary for the
caller.  As TDB should be refcounted, it makes not sense to always
return it.  Pass an output pointer for the TDB which can be NULL.
OK mvs@ tobhe@