rooted.js: TypeError

If I drag the root way off the tree into the lower-left corner of the SVG, I get the following error:
```js
Uncaught TypeError: Cannot read property 'x1' of undefined
    at closestEdge (rooting.js:215)
    at SVGRectElement.<anonymous> (rooting.js:146)
    at SVGRectElement.__onmousemove (d3.v3.min.js:1)
```
Animation proceeds without other problems.