Bug 750625 - Fix Coverity (11060) Dereference null return value
    
    https://bugzilla.redhat.com/show_bug.cgi?id=750625
    
    slapd/back-ldbm/ldbm_modrdn.c (moddn_rename_child_entry)
    
    Bug Description: Dereferencing a null pointer "olddns".
    Missing a check of the NULL possibilities for olddns and olddn.
    
    Fix Description: check if olddn and olddns are NULL or not.
    If NULL, it does not go forward, but returns.