path: fix path.normalize for relative paths

After slicing, the `lastSegmentLength` should be calculated again,
instead of assigning value `j`.

PR-URL: https://github.com/nodejs/node/pull/17974
Fixes: https://github.com/nodejs/node/issues/17928


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>