Use `\0` instead of `\x00`

Diff: https://github.com/mbostock/d3/pull/1569/files?w=1

In this case, `\0` can safely be used instead of `\x00`, since there are no digits immediately following it.

The UglifyJS output for `d3.js` is fine in either case. 

It seems the change in c6f9932035449a7efe942b55aa57a0eccf6b4fe7 was done by accident or because of a misunderstanding.

Ref. https://github.com/mbostock/d3/commit/c6f9932035449a7efe942b55aa57a0eccf6b4fe7