Path traversal on Windows in path/filepath

A path traversal vulnerability exists in filepath.Clean on Windows. On Windows, the filepath.Clean function could transform an invalid path such as "a/../c:/b" into the valid path "c:\b". This transformation of a relative (if invalid) path into an absolute path could enable a directory traversal attack. After fix, the filepath.Clean function transforms this path into the relative (but still invalid) path ".\c:\b".