windows: avoid length overflow in NewNTString

NewNTString initializes an NTString with RtlInitString, which cannot accurately represent sources whose NUL-terminated byte length exceeds MAX_USHORT. Reject strings whose byte representation cannot fit in the uint16 length fields before calling RtlInitString.

Add a boundary round-trip test for NTString.

Fixes golang/go#80103

Change-Id: Id38882a952330305e6c23d0adc86a8bf7920d30e
Reviewed-on: https://go-review.googlesource.com/c/sys/+/793100
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>