src: do not leak handles on debug and exit

Ensure HandleScope is created before creating any new handles (which
`Context::Scope` and `Environment::GetCurrent` does).

PR-URL: https://github.com/iojs/io.js/pull/1133


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Brian White <mscdex@mscdex.net>