syscall: optimize UTF16{,Ptr}FromString

Use bytealg.IndexByteString in UTF16FromString instead of an open-coded
loop.

Change-Id: I366448382f2d0adeca6b254131e0087a1f489258
Reviewed-on: https://go-review.googlesource.com/c/go/+/393614
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/syscall/syscall_windows.go

@@ -8,6 +8,7 @@ package syscall
 
 import (
 	errorspkg "errors"
+	"internal/bytealg"
 	"internal/itoa"
 	"internal/oserror"
 	"internal/race"
@@ -39,10 +40,8 @@ func StringToUTF16(s string) []uint16 {
 // s, with a terminating NUL added. If s contains a NUL byte at any
 // location, it returns (nil, EINVAL).
 func UTF16FromString(s string) ([]uint16, error) {
-	for i := 0; i < len(s); i++ {
-		if s[i] == 0 {
-			return nil, EINVAL
-		}
+	if bytealg.IndexByteString(s, 0) != -1 {
+		return nil, EINVAL
 	}
 	return utf16.Encode([]rune(s + "\x00")), nil
 }