avoid cross device

avoid dir nil.
if dir is nil, writeAtomic creatae tempfile in system temporary dir and cross device

ioutil2/ioutil.go

@@ -13,7 +13,7 @@ import (
 
 // Write file to temp and atomically move when everything else succeeds.
 func WriteFileAtomic(filename string, data []byte, perm os.FileMode) error {
-	dir, name := path.Split(filename)
+	dir, name := path.Dir(filename), path.Base(filename) 
 	f, err := ioutil.TempFile(dir, name)
 	if err != nil {
 		return err