Prior to this commit, uos.chdir('/') followed by uos.stat('noexist') would
succeed that stat even though the entry did not exist (some other functions
like listdir would have similar issues). This is because, if the current
directory was the root and the path was relative, mp_vfs_lookup_path would
return success for bad paths.
Signed-off-by: Damien George <damien@micropython.org>
