syscall: fix bounds check in Error

Fixes #3042.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/5675067

src/pkg/syscall/syscall_unix.go

@@ -95,7 +95,7 @@ func (m *mmapper) Munmap(data []byte) (err error) {
 type Errno uintptr
 
 func (e Errno) Error() string {
-	if 0 <= e && int(e) < len(errors) {
+	if 0 <= int(e) && int(e) < len(errors) {
 		s := errors[e]
 		if s != "" {
 			return s