Ticket 590 - ns-slapd segfaults while trying to delete a tombstone entry
    
    Bug Description:  While trying to remove a tombstone entry, ns-slapd
                      crashed with segfault.
    
    Fix Description:  Check if op_parms->csn is NULL before dereferencing.
                      This was fixed in master via ticket 532, but this
                      patch is just part of that fix to avoid the crash.
    
    https://fedorahosted.org/389/ticket/590
    
    Reviewed by: Noriko(Thanks!)
    (cherry picked from commit 39f19ae08afe28eb36eabe76b2add1e84c7ed805)