Bug 750625 - Fix Coverity (12195) Dereference after null check
    
    https://bugzilla.redhat.com/show_bug.cgi?id=750625
    
    slapd/dn.c (slapi_dn_normalize_ext)
    
    Bug Description: Dereferencing null variable "dest_len".
    Missed to check the NULL possibility of the argument "dest_len"
    to return the normalized dn size.
    
    Fix Description: Assinging 0 to "*dest_len" only when "dest_len"
    is not NULL.