windows: fix slicing of NTUnicodeString values

We were slicing using a count of bytes, not a count of uint16s.

Fixes golang/go#73460

Change-Id: If0fd19e795078c01fda5b976e3c34af115b25dcc
Reviewed-on: https://go-review.googlesource.com/c/sys/+/667235
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Auto-Submit: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@google.com>