fs: return null error on readFile() success

This commit ensures that readFile() callsback with a null
error consistently on success.

PR-URL: https://github.com/nodejs/node/pull/3740


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>