cmd/gotext: fix "go format failed: invalid argument"

Fixes golang/go#26897

Change-Id: Ie47e648f02f2b1ed20e72ab6a05035f91e4635fc
Reviewed-on: https://go-review.googlesource.com/128795
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

cmd/gotext/rewrite.go

@@ -5,6 +5,7 @@
 package main
 
 import (
+	"io"
 	"os"
 
 	"golang.org/x/text/message/pipeline"
@@ -39,9 +40,9 @@ using Printf to allow translators to reorder arguments.
 }
 
 func runRewrite(cmd *Command, _ *pipeline.Config, args []string) error {
-	w := os.Stdout
-	if *overwrite {
-		w = nil
+	var w io.Writer
+	if !*overwrite {
+		w = os.Stdout
 	}
 	pkg := "."
 	switch len(args) {