s4:schema_load LDB module - fix a segfault condition on schema refresh

s4:schema_load LDB module - fix a segfault condition on schema refresh

The schema refresh operation itself starts requests from the top of the LDB
modules stack (see call "dsdb_schema_set_attributes" - search operations).
This doesn't work well when these do perform "dsdb_get_schema" calls. Since the
new schema isn't marked as "refreshed" atm (but in fact it still is - we didn't
terminate the reload/refresh yet) we could perform other calls to
"dsdb_schema_refresh" and run into serious trouble (segfault).