path: fix normalize for absolutes

Fixes a regression introduced by
b212be08.

path.normalize(''/a/b/c/../../../x/y/z'') should return '/x/y/z'.

Fixes: https://github.com/nodejs/node/issues/5585
PR-URL: https://github.com/nodejs/node/pull/5589


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>