cmd/gofmt: don't leave tmp file if -w failed

Follow-up on https://golang.org/cl/33018.

For #8984.

Change-Id: I6655a5537a60d4ea3ee13029a56a75b150f8c8f8
Reviewed-on: https://go-review.googlesource.com/33020
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/gofmt/gofmt.go

@@ -243,15 +243,17 @@ func diff(b1, b2 []byte) (data []byte, err error) {
 // implemented.
 func writeFile(filename string, data []byte, perm os.FileMode) error {
 	// open temp file
-	f, err := ioutil.TempFile(filepath.Dir(filename), "tmp")
+	f, err := ioutil.TempFile(filepath.Dir(filename), "gofmt-")
 	if err != nil {
 		return err
 	}
+	tmpname := f.Name()
 	err = f.Chmod(perm)
 	if err != nil {
+		f.Close()
+		os.Remove(tmpname)
 		return err
 	}
-	tmpname := f.Name()
 
 	// write data to temp file
 	n, err := f.Write(data)
@@ -261,9 +263,12 @@ func writeFile(filename string, data []byte, perm os.FileMode) error {
 	if err1 := f.Close(); err == nil {
 		err = err1
 	}
+	if err == nil {
+		err = os.Rename(tmpname, filename)
+	}
 	if err != nil {
-		return err
+		os.Remove(tmpname)
 	}
 
-	return os.Rename(tmpname, filename)
+	return err
 }