Join("C:", "", "b") must return relative path "C:b"

Fixes #26953

Change-Id: I2f843ce3f9f18a1ce0e2d0f3a15233f237992776
Reviewed-on: https://go-review.googlesource.com/129758
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>