Bug 750624 - Fix Coverity (11053) Explicit null dereferenced:
                 slapi_dn_normalize_ext (slapd/dn.c)
    
    https://bugzilla.redhat.com/show_bug.cgi?id=750624
    
    Bug Description: Dereferencing null variable "d".
    There is no possibility that (rc > 0) && (d == NULL), but it's
    safe to check if d is not NULL before assigning '\0' to *d.
    
    Fix Description: add checking if "d" is NULL or not before
    assigning '\0' to *d.