tz: initialize digest in Sum

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

tz/digest.go

@@ -29,6 +29,7 @@ func New() *digest {
 // Sum returns Tillich-Zémor checksum of data.
 func Sum(data []byte) [Size]byte {
 	d := new(digest)
+	d.Reset()
 	_, _ = d.Write(data) // no errors
 	return d.checkSum()
 }

tz/hash_test.go

@@ -151,6 +151,7 @@ func TestHomomorphism(t *testing.T) {
 
 	// Test if our hashing is really homomorphic
 	h = Sum(b)
+	require.NotEqual(t, [64]byte{}, h)
 	h1 = Sum(b[:32])
 	h2 = Sum(b[32:])