src: fixes misplaced comment

In e26622bd, a comment was incorrectly moved from the code
it was describing.

Fixes: https://github.com/nodejs/node/issues/8856
PR-URL: https://github.com/nodejs/node/pull/8860


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>