path: Fix joining Windows path when the receiver is "C:"

WindowsPath::new("C:").join("a") produces r"C:\a". This is incorrect.
It should produce "C:a".
auto merge of #11579 : kballard/rust/windows-path-join, r=erickt

WindowsPath::new("C:").join("a") produces r"C:". This is incorrect.
It should produce "C:a".