addControlDescriptions would panic at control without critcality and value

[vetinari]

a fix similar to #83 should be applied here also (ldap.go)

func addControlDescriptions(packet *ber.Packet) {
    packet.Description = "Controls"
    for _, child := range packet.Children {
        child.Description = "Control"
        child.Children[0].Description = "Control Type (" + ControlDescription(child.Children[0].Value.(string)) + ")"
        value := child.Children[1]
        if len(child.Children) == 3 {
            child.Children[1].Description = "Criticality"
            value = child.Children[2]
        }
        value.Description = "Control Value"