(ITS#7303) bdb/hdb search checks aliases unnecessarily

[hyc@OpenLDAP.org]

Full_Name: Howard Chu
Version: HEAD/RE24
OS: 
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (37.19.96.193)
Submitted by: hyc


If a search request has derefSearching set in the alias deref option, the
search_aliases() function walks thru a (potentially) large number of entries
checking to see if they are aliases, even if the objectclass index shows there
are no alias entries in the database. It should exit early instead.

The bug is also present to a lesser degree in back-mdb; the back-mdb version of
search_aliases() would only do a single unneeded entry lookup.