process: fix incorrect usage of assert.fail()

The message argument for `assert.fail()` is the third argument, not the
first. Correct minor misuse in internal module.

PR-URL: https://github.com/nodejs/node/pull/6211


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>