fix precision error on voronoi.neighbors

[Fil]

closes #137

Note that the test

d3-delaunay/test/voronoi-test.js

Line 54 in 2196b80

it("voronoi neighbors are clipped", () => {

was in the same situation (a point completely in the corner), and was testing the wrong result 😰

fix built at https://observablehq.com/@d3/voronoi-neighbors-137

cc: @UnluckyNinja

[mbostock]

I improved the formatting. 😬 I wish I knew how to implement this robustly like delaunator does…