fs: ensure nullCheck() callback is a function

Currently, nullCheck() will attempt to invoke any truthy value
as a function if the path argument contains a null character.
This commit validates that the callback is actually a function
before trying to invoke it. fs.access() was vulnerable to this
bug, as nullCheck() was called prior to type checking its
callback.

PR-URL: https://github.com/iojs/io.js/pull/887


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