Security fix, CVE-2017-14493, DHCPv6 - Stack buffer overflow.
    
    Fix stack overflow in DHCPv6 code. An attacker who can send
    a DHCPv6 request to dnsmasq can overflow the stack frame and
    crash or control dnsmasq.
    
    Signed-off-by: Petr Menšík <pemensik@redhat.com>