windows: fix -d=checkptr slice failures

This CL fixes unsafe casts to slices that are missing length or capacity.

Running tests with -d=checkptr enabled may panic on casting unsafe.Pointer to a static array of large predefined length, that is most likely much bigger than the size of the actual array in memory. Checkptr check is not satisfied if slicing operator misses length and capacity arguments `(*[(1 << 30) - 1]uint16)(unsafe.Pointer(p))[:]`, or when there is no slicing at all `(*[(1 << 30) - 1]uint16)(unsafe.Pointer(p))`.

To find all potential cases I used `grep -nr ")(unsafe.Pointer(" ./windows`, then filtered out safe casts when object size is always static and known at compile time.

To reproduce the issue run tests with checkptr enabled `go test -a -gcflags=all=-d=checkptr ./windows/...`.

Updates golang/go#34972
Fixes golang/go#38355

Change-Id: I9dd2084b4f9fb7618cdb140fb2f38b56b6d6cc04
GitHub-Last-Rev: 73288ad18a2692b1510f8ed564daad8d1cbd7e59
GitHub-Pull-Request: golang/sys#65
Reviewed-on: https://go-review.googlesource.com/c/sys/+/225418
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

windows/svc/security.go

@@ -7,8 +7,6 @@
 package svc
 
 import (
-	"unsafe"
-
 	"golang.org/x/sys/windows"
 )
 
@@ -48,9 +46,8 @@ func IsAnInteractiveSession() (bool, error) {
 	if err != nil {
 		return false, err
 	}
-	p := unsafe.Pointer(&gs.Groups[0])
-	groups := (*[2 << 20]windows.SIDAndAttributes)(p)[:gs.GroupCount]
-	for _, g := range groups {
+
+	for _, g := range gs.AllGroups() {
 		if windows.EqualSid(g.Sid, interSid) {
 			return true, nil
 		}