Confine the upload path

Implement resolve_upload(base, name) returning the joined, normalized path for a user-supplied file name — or raising ValueError('traversal') when the name: - is empty, absolute (/etc/passwd), or contains a NUL character - contains any .. component (../../etc/passwd) - would resolve outside base after joining Valid names ('report.pdf', 'sub/dir/img.png') resolve to base/name strings.

Difficulty: advanced