Dir(`\\server\share`) returns `\\server\share.`. Change Dir so it
returns `\\server\share` instead.

Fixes #18783

Change-Id: I9e0dd71ea6aea85e6c6114aaa4bb3bea3270d818
Reviewed-on: https://go-review.googlesource.com/35690Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>