NaN in low-segment spline

[cmyr]

The following test case fails. I haven't dug into this at all.

    #[test]
    fn line_to_spline_to_crash() {
        let mut spec = SplineSpec::new();
        spec.move_to(Point::new(0., 0.));
        spec.spline_to(Some(Point::new(100.0, 0.0)), None, Point::new(0.0, 100.0), true);
        spec.line_to(Point::new(0.0, 0.0), false);
        for segment in spec.solve().segments() {
            assert!(segment.p1.x.is_normal(), segment.p2.x.is_normal());
        }
    }