Avoid backtracking in v2

[timbset]

Fixes https://github.com/d3/d3-color/issues/108

As the main branch already contains v3 and this PR was created to fix a security issue in v2, I suggest you to create a new branch called "v2" based on the following commit: https://github.com/d3/d3-color/commit/017a46380fd46600c1781da634039478c0b23e13. If you agree, please change base branch in this PR to the created one.

[vallieres]

@mbostock @Fil sorry to ping you, but this PR would come in and help lots of us on the 2.x track and fix a troublesome CVE 😄

[Fernando-Abreu]

It would be really helpful to has this issue fixed in version 2.x

[vignesh-harish]

@mbostock - sorry to bother you, but is this something that you can help merge ?