types: Hash invoked for nil Data and Header should not panic

Fixes https://github.com/tendermint/tendermint/issues/1298
Fixes https://github.com/tendermint/tendermint/issues/1299

Found while writing tests in https://github.com/tendermint/tendermint/pull/1300

types/block.go

@@ -179,7 +179,7 @@ type Header struct {
 // Hash returns the hash of the header.
 // Returns nil if ValidatorHash is missing.
 func (h *Header) Hash() cmn.HexBytes {
-	if len(h.ValidatorsHash) == 0 {
+	if h == nil || len(h.ValidatorsHash) == 0 {
 		return nil
 	}
 	return merkle.SimpleHashFromMap(map[string]merkle.Hasher{
@@ -413,6 +413,9 @@ type Data struct {
 
 // Hash returns the hash of the data
 func (data *Data) Hash() cmn.HexBytes {
+	if data == nil {
+		return (Txs{}).Hash()
+	}
 	if data.hash == nil {
 		data.hash = data.Txs.Hash() // NOTE: leaves of merkle tree are TxIDs
 	}

types/block_test.go

@@ -3,7 +3,9 @@ package types
 import (
 	"testing"
 
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+
 	crypto "github.com/tendermint/go-crypto"
 	cmn "github.com/tendermint/tmlibs/common"
 )
@@ -73,3 +75,15 @@ func makeBlockID(hash string, partSetSize int, partSetHash string) BlockID {
 	}
 
 }
+
+var nilBytes []byte
+
+func TestNilHeaderHashDoesntCrash(t *testing.T) {
+	assert.Equal(t, []byte((*Header)(nil).Hash()), nilBytes)
+	assert.Equal(t, []byte((new(Header)).Hash()), nilBytes)
+}
+
+func TestNilDataHashDoesntCrash(t *testing.T) {
+	assert.Equal(t, []byte((*Data)(nil).Hash()), nilBytes)
+	assert.Equal(t, []byte(new(Data).Hash()), nilBytes)
+}