NaN in low-segment spline

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

```rust
#[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());
}
}
```

Start with the failing line_to_spline_to_crash test and trace SplineSpec::solve through the spline_to and line_to calls. Reproduce the case, identify why a segment coordinate becomes NaN, and consider the work done when the test passes with all segment coordinates normal.

Written by the indexing model from the issue text.