[release-branch.go1.19] syscall: fix invalid unsafe.Pointer conversion on Windows

This cherry-pick CL 471335 without using unsafe.{Add,Slice}.

Fixes #58773

Change-Id: Ifa5c059ed5e358ed98aee7e83b95dd1806b535f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/471335
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/471935
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>

src/syscall/env_windows.go

@@ -68,21 +68,25 @@ func Clearenv() {
 }
 
 func Environ() []string {
-	s, e := GetEnvironmentStrings()
+	envp, e := GetEnvironmentStrings()
 	if e != nil {
 		return nil
 	}
-	defer FreeEnvironmentStrings(s)
+	defer FreeEnvironmentStrings(envp)
+
 	r := make([]string, 0, 50) // Empty with room to grow.
-	for from, i, p := 0, 0, (*[1 << 24]uint16)(unsafe.Pointer(s)); true; i++ {
-		if p[i] == 0 {
-			// empty string marks the end
-			if i <= from {
-				break
-			}
-			r = append(r, string(utf16.Decode(p[from:i])))
-			from = i + 1
+	const size = unsafe.Sizeof(*envp)
+	for *envp != 0 { // environment block ends with empty string
+		// find NUL terminator
+		end := unsafe.Pointer(envp)
+		for *(*uint16)(end) != 0 {
+			end = unsafe.Pointer(uintptr(end) + size)
 		}
+
+		n := (uintptr(end) - uintptr(unsafe.Pointer(envp))) / size
+		entry := (*[(1 << 30) - 1]uint16)(unsafe.Pointer(envp))[:n:n]
+		r = append(r, string(utf16.Decode(entry)))
+		envp = (*uint16)(unsafe.Pointer(uintptr(end) + size))
 	}
 	return r
 }