Fix initial duplicate detection.
Fix initial duplicate detection.

When detecting coincident points, the previous site [xsitex, xsitey] should be
initialized to an illegal value (different from any potential valid input site).
But [Number.MIN_VALUE, Number.MIN_VALUE] is a valid site, so use [undefined,
undefined] instead.

gorhill/Javascript-Voronoi,mbostock/Javascript-Voronoi,jjviscomi/Javascript-Voronoi,edrohler/Javascript-Voronoi,edrohler/Javascript-Voronoi,edrohler/Javascript-Voronoi,mbostock/Javascript-Voronoi,jjviscomi/Javascript-Voronoi,gorhill/Javascript-Voronoi,gorhill/Javascript-Voronoi,jjviscomi/Javascript-Voronoi