network: fix MerkleBlock serialization

1. It contains `block.Base` thus needs network magic.
2. TxCount should match number of hashes.

pkg/network/message.go

@@ -159,7 +159,7 @@ func (m *Message) decodePayload() error {
 	case CMDTX:
 		p = &transaction.Transaction{Network: m.Network}
 	case CMDMerkleBlock:
-		p = &payload.MerkleBlock{}
+		p = &payload.MerkleBlock{Network: m.Network}
 	case CMDPing, CMDPong:
 		p = &payload.Ping{}
 	case CMDNotFound:

pkg/network/payload/merkleblock.go

@@ -1,6 +1,9 @@
 package payload
 
 import (
+	"errors"
+
+	"github.com/nspcc-dev/neo-go/pkg/config/netmode"
 	"github.com/nspcc-dev/neo-go/pkg/core/block"
 	"github.com/nspcc-dev/neo-go/pkg/io"
 	"github.com/nspcc-dev/neo-go/pkg/util"
@@ -9,6 +12,7 @@ import (
 // MerkleBlock represents a merkle block packet payload.
 type MerkleBlock struct {
 	*block.Base
+	Network netmode.Magic
 	TxCount int
 	Hashes  []util.Uint256
 	Flags   []byte
@@ -16,7 +20,7 @@ type MerkleBlock struct {
 
 // DecodeBinary implements Serializable interface.
 func (m *MerkleBlock) DecodeBinary(br *io.BinReader) {
-	m.Base = &block.Base{}
+	m.Base = &block.Base{Network: m.Network}
 	m.Base.DecodeBinary(br)
 
 	txCount := int(br.ReadVarUint())
@@ -26,6 +30,9 @@ func (m *MerkleBlock) DecodeBinary(br *io.BinReader) {
 	}
 	m.TxCount = txCount
 	br.ReadArray(&m.Hashes, m.TxCount)
+	if txCount != len(m.Hashes) {
+		br.Err = errors.New("invalid tx count")
+	}
 	m.Flags = br.ReadVarBytes((txCount + 7) / 8)
 }