core: do not rewrite binreader error for bad Conflicts attr

We should keep the original BinReader error untouched in case if
it is exists.

pkg/core/transaction/attribute_test.go

@@ -7,6 +7,8 @@ import (
 
 	"github.com/nspcc-dev/neo-go/internal/random"
 	"github.com/nspcc-dev/neo-go/internal/testserdes"
+	"github.com/nspcc-dev/neo-go/pkg/io"
+	"github.com/nspcc-dev/neo-go/pkg/util"
 	"github.com/stretchr/testify/require"
 )
 
@@ -61,6 +63,7 @@ func TestAttribute_EncodeBinary(t *testing.T) {
 		})
 	})
 	t.Run("Conflicts", func(t *testing.T) {
+		t.Run("positive", func(t *testing.T) {
 			attr := &Attribute{
 				Type: ConflictsT,
 				Value: &Conflicts{
@@ -69,6 +72,17 @@ func TestAttribute_EncodeBinary(t *testing.T) {
 			}
 			testserdes.EncodeDecodeBinary(t, attr, new(Attribute))
 		})
+		t.Run("negative: too long", func(t *testing.T) {
+			bw := io.NewBufBinWriter()
+			bw.WriteVarBytes(make([]byte, util.Uint256Size+1))
+			require.Error(t, testserdes.DecodeBinary(bw.Bytes(), new(Conflicts)))
+		})
+		t.Run("negative: bad uint256", func(t *testing.T) {
+			bw := io.NewBufBinWriter()
+			bw.WriteVarBytes(make([]byte, util.Uint256Size-1))
+			require.Error(t, testserdes.DecodeBinary(bw.Bytes(), new(Conflicts)))
+		})
+	})
 }
 
 func TestAttribute_MarshalJSON(t *testing.T) {

pkg/core/transaction/conflicts.go

@@ -12,7 +12,11 @@ type Conflicts struct {
 
 // DecodeBinary implements io.Serializable interface.
 func (c *Conflicts) DecodeBinary(br *io.BinReader) {
-	hash, err := util.Uint256DecodeBytesBE(br.ReadVarBytes(util.Uint256Size))
+	bytes := br.ReadVarBytes(util.Uint256Size)
+	if br.Err != nil {
+		return
+	}
+	hash, err := util.Uint256DecodeBytesBE(bytes)
 	if err != nil {
 		br.Err = err
 		return