path: fix win32.isAbsolute()

Pull Request check-list

• Does make -j8 test (UNIX) or vcbuild test nosign (Windows) pass with this change (including linting)?
• Is the commit message formatted according to CONTRIBUTING.md?
• If this change fixes a bug (or a performance problem), is a regression test (or a benchmark) included?

Affected core subsystem(s)

• path

Description of change

Fixes: https://github.com/nodejs/node/issues/6027