cmd/shfmt: handle errors properly in filepath.Walk

If the provided error is non-nil, info might be nil. We should check the
error first.

In practice, this only triggered with weird corner cases such as running
'shfmt -f /' inside a container, since some special files would give
weird I/O errors. As such, it's hard to write a small test for this.

The new code seems logical, and it's unlikely we'll break it again, so
don't add a test for now.

Fixes #466.

cmd/shfmt/main.go

@@ -208,13 +208,13 @@ func walk(path string, onError func(error)) {
 		return
 	}
 	filepath.Walk(path, func(path string, info os.FileInfo, err error) error {
-		if info.IsDir() && vcsDir.MatchString(info.Name()) {
-			return filepath.SkipDir
-		}
 		if err != nil {
 			onError(err)
 			return nil
 		}
+		if info.IsDir() && vcsDir.MatchString(info.Name()) {
+			return filepath.SkipDir
+		}
 		conf := fileutil.CouldBeScript(info)
 		if conf == fileutil.ConfNotScript {
 			return nil