cmd/gotext: fix segfault in extract & rewrite commands

If extract or rewrite are called with no arguments it results in a
segmentation fault, since we dereference the out flag that was
not defined for these commands.

Fixes golang/go#62697

Change-Id: I697943b7c221431d0361bcec74c18183f6e141ea
GitHub-Last-Rev: 313ddfa
GitHub-Pull-Request: #46
Reviewed-on: https://go-review.googlesource.com/c/text/+/529255
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

cmd/gotext/main.go

@@ -48,11 +48,18 @@ func config() (*pipeline.Config, error) {
 	if err != nil {
 		return nil, wrap(err, "invalid srclang")
 	}
+
+	// Use a default value since rewrite and extract don't have an out flag.
+	genFile := ""
+	if out != nil {
+		genFile = *out
+	}
+
 	return &pipeline.Config{
 		SourceLanguage:      tag,
 		Supported:           getLangs(),
 		TranslationsPattern: `messages\.(.*)\.json$`,
-		GenFile:             *out,
+		GenFile:             genFile,
 		Dir:                 *dir,
 	}, nil
 }