windows: avoid uint16 overflow in NewNTUnicodeString

Fixes CVE-2026-39824
Fixes #78916

Change-Id: I344518a17d59fd81c4bb39da0b3e13be6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/sys/+/770080
Reviewed-by: Neal Patel <nealpatel@google.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>