windows: fix parsing of non-ASCII entries in token.Environ

Fixes golang/go#65055, the unexpected behavior of token.Environ in parsing entries containing runes larger than 2 bytes in size

Change-Id: I753d2c605e3a2d7a1d90cd18601d6b918f0d3f7a
Reviewed-on: https://go-review.googlesource.com/c/sys/+/556895
Auto-Submit: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>

windows/env_windows.go

@@ -37,14 +37,17 @@ func (token Token) Environ(inheritExisting bool) (env []string, err error) {
 		return nil, err
 	}
 	defer DestroyEnvironmentBlock(block)
-	blockp := unsafe.Pointer(block)
-	for {
-		entry := UTF16PtrToString((*uint16)(blockp))
-		if len(entry) == 0 {
-			break
+	size := unsafe.Sizeof(*block)
+	for *block != 0 {
+		// find NUL terminator
+		end := unsafe.Pointer(block)
+		for *(*uint16)(end) != 0 {
+			end = unsafe.Add(end, size)
 		}
-		env = append(env, entry)
-		blockp = unsafe.Add(blockp, 2*(len(entry)+1))
+
+		entry := unsafe.Slice(block, (uintptr(end)-uintptr(unsafe.Pointer(block)))/size)
+		env = append(env, UTF16ToString(entry))
+		block = (*uint16)(unsafe.Add(end, size))
 	}
 	return env, nil
 }