vm: fix OOM during nested structure cloning

Resulting item can't have more than MaxStackSize elements. Technically this
limits to MaxStackSize cloned elements but it's considered to be enough to
mitigate the issue (the next size check is going to happen during push to the
stack). See neo-project/neo#2534, thanks @vang1ong7ang.

pkg/vm/stackitem/item_test.go

@@ -465,6 +465,15 @@ func TestNewVeryBigInteger(t *testing.T) {
 	check(false, new(big.Int).Mul(maxBitSet, big.NewInt(2)))
 }
 
+func TestStructClone(t *testing.T) {
+	st0 := Struct{}
+	st := Struct{value: []Item{&st0}}
+	_, err := st.Clone(1)
+	require.NoError(t, err)
+	_, err = st.Clone(0)
+	require.Error(t, err)
+}
+
 func TestDeepCopy(t *testing.T) {
 	testCases := []struct {
 		name string