Ticket 47427 - Overflow in nsslapd-disk-monitoring-threshold
    
    Bug Description:  If you directory try and set the threshold to -2,
                      the server allows it, but it should be rejected.
    
    Fix Description:  Use a signed int when parsing the value.
    
    https://fedorahosted.org/389/ticket/47427
    
    Reviewed by: richm(Thanks!)
    (cherry picked from commit 0b30980695509347d8f883ac4405fa727041269f)