compress/gzip: add missing error check in test

Change-Id: Ib7be503a46f3609ea29464569e57e1f620e97a53
GitHub-Last-Rev: a43cffe45f3b9d208028f7682088cbe537d6deca
GitHub-Pull-Request: golang/go#30007
Reviewed-on: https://go-review.googlesource.com/c/go/+/160431
Run-TryBot: Leon Klingele <git@leonklingele.de>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/compress/gzip/gzip_test.go

@@ -214,6 +214,9 @@ func TestConcat(t *testing.T) {
 	w.Close()
 
 	r, err := NewReader(&buf)
+	if err != nil {
+		t.Fatal(err)
+	}
 	data, err := ioutil.ReadAll(r)
 	if string(data) != "hello world\n" || err != nil {
 		t.Fatalf("ReadAll = %q, %v, want %q, nil", data, err, "hello world")