encoding/hex: fix Dumper not always closing on Close call

Updates #23574

Change-Id: I1b87390679e0817a2f6e4e5938994ea32df87bd7
Reviewed-on: https://go-review.googlesource.com/107596Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/hex/hex.go

@@ -282,10 +282,13 @@ func (h *dumper) Write(data []byte) (n int, err error) {
 
 func (h *dumper) Close() (err error) {
 	// See the comments in Write() for the details of this format.
-	if h.used == 0 || h.closed {
+	if h.closed {
 		return
 	}
 	h.closed = true
+	if h.used == 0 {
+		return
+	}
 	h.buf[0] = ' '
 	h.buf[1] = ' '
 	h.buf[2] = ' '

src/encoding/hex/hex_test.go

@@ -204,6 +204,19 @@ func TestDumper_doubleclose(t *testing.T) {
 	}
 }
 
+func TestDumper_earlyclose(t *testing.T) {
+	var out bytes.Buffer
+	dumper := Dumper(&out)
+
+	dumper.Close()
+	dumper.Write([]byte(`gopher`))
+
+	expected := ""
+	if out.String() != expected {
+		t.Fatalf("got:\n%#v\nwant:\n%#v", out.String(), expected)
+	}
+}
+
 func TestDump(t *testing.T) {
 	var in [40]byte
 	for i := range in {