plan9: use bytes.IndexByte instead of a for loop

Change-Id: Iae896dc32e775cb469fc3d0d367fd7c7825161ec
Reviewed-on: https://go-review.googlesource.com/99516
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

plan9/syscall_plan9.go

@@ -12,6 +12,7 @@
 package plan9
 
 import (
+	"bytes"
 	"syscall"
 	"unsafe"
 )
@@ -50,12 +51,11 @@ func atoi(b []byte) (n uint) {
 }
 
 func cstring(s []byte) string {
-	for i := range s {
+	i := bytes.IndexByte(s, 0)
-		if s[i] == 0 {
+	if i == -1 {
-			return string(s[0:i])
+		i = len(s)
-		}
 	}
-	return string(s)
+	return string(s[:i])
 }
 
 func errstr() string {