src: fix negative values in process.hrtime()

Fix a regression introduced in commit 89f056bd ("node: improve
performance of hrtime()") where the nanosecond field sometimes
had a negative value when calculating the difference between two
timestamps.

Fixes: https://github.com/nodejs/node/issues/4751
PR-URL: https://github.com/nodejs/node/pull/4757


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>