path: fix win32 parse()

Fix path.win32.parse("/foo/bar") retuns `{root: '' ...}`(v5.7.0),
but not `{root: '/' ...}`(v5.6.0).

PR-URL: https://github.com/nodejs/node/pull/5484


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Roman Reiss <me@silverwind.io>

Conflicts:
	test/parallel/test-path-parse-format.js