fixed unexpected reflect behavior

After tls.Unmarshal() an empty slice is not assigned the nil value, but rather
a slice of zero length.  It is in contrast to NewTreeHeadV1(), which assigns a
nil value.  Therefore, reflect.DeepEqual() considers them to be different.

Fixed by assigning an empty tree head extension as `make([]byte, 0)`,
and not looking for nil values but rather zero-length values with len().

Further read: "Note that a non-nil empty slice and a nil slice [...] are
not deeply equal." (https://golang.org/pkg/reflect/#DeepEqual).

1 files changed, 2 insertions, 2 deletions

diff --git a/crypto_test.go b/crypto_test.go
index cfbb0a8..75e530e 100644
--- a/crypto_test.go
+++ b/crypto_test.go
@@ -118,8 +118,8 @@ func TestGenV1Sth(t *testing.T) {
 		if got, want := sth.TreeHead.RootHash.Data, th.RootHash.Data; !bytes.Equal(got, want) {
 			t.Errorf("got root hash %X, wanted %X in test %q", got, want, table.description)
 		}
-		if sth.TreeHead.Extension != nil {
-			t.Errorf("got extensions %X, wanted nil in test %q", sth.TreeHead.Extension, table.description)
+		if len(sth.TreeHead.Extension) != 0 {
+			t.Errorf("got extensions %X, wanted none in test %q", sth.TreeHead.Extension, table.description)
 		}
 	}
 }