node: set process._eventsCount to 0 on startup

process is an EventEmitter. There are operations that increment and
decrement the _eventsCount property of an EventEmitter.
process._eventsCount would previously get set to NaN. This change makes
process._eventsCount be calculated as expected.

PR-URL: https://github.com/nodejs/node/pull/5208


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>