internal/syscall/windows: simplify unsafe.Slice usage
internal/syscall/windows: simplify unsafe.Slice usage

CL 428780 used unsafe.Slice instead of unsafeheader for simplifiying the
code. However, it can be even simpler, since "p" is already a *uin16,
the unsafe cast is not necessary.

Change-Id: Idc492b73518637997e85c0b33f8591bd19b7929f
Reviewed-on: https://go-review.googlesource.com/c/go/+/429915
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Auto-Submit: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Reviewed-by: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go