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.
Merge pull request #14 from mbostock/fix-initial-duplicate

Fix initial duplicate detection.
updated to reflect issue #14 fix in non minified version