contextify: use CHECK instead of `if`

I was walking through the vm changes and saw this particular `if` check
interesting. In case `ctx` is empty it's going to fail later anyways.
So, instead of putting an `if` check there;

option a - use CHECK
option b - do nothing

Considering the developer wanted to make sure `ctx` is not empty,
`CHECK` option looked more convenient.

PR-URL: https://github.com/nodejs/node/pull/3125


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>