Bug 750625 - Fix Coverity (12196) Dereference before null check
    
    https://bugzilla.redhat.com/show_bug.cgi?id=750625
    
    slapd/back-ldbm/ldbm_entryrdn.c (entryrdn_lookup_dn)
    
    Bug Description: Dreferencing "db" before a null check.
    
    Fix Description: Adding a check if the variable "db" returned
    from _entryrdn_open_index is NULL or not.  If NULL, it returns
    or goto bail there.