Update util.py: Fix join_path function

This change alters the meaning of this loop entirely, as any token (to be appended) which looks like an absolute path will completely replace the path concatenated so far.
The original intention of this function is to treat every path token (`b`) as relative path, which is what this portion tries to accomplish. 
Unfortunately I don't know the reason for doing this, as nowadays I would prefer an assertion as all tokens should be relative.