src: refactor contextify

Small refactoring to make contextify more readable.
Remove auto and inline FromJust(). Simplify
if statement.

PR-URL: https://github.com/nodejs/node/pull/8909


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>