vm: fix incorrect refcounting in POPITEM

We're popping an item (array) off the stack, OK, it triggers refs.Remove() for
it. Then we're pushing an inner item to the stack, OK, it triggers refs.Add()
for this element. Why are we removing it afterwards? Looks like something went
wrong in 324107b31e (and https://github.com/nspcc-dev/neo-go/pull/1670)
since a simple test shows zero counter after POPITEM and -1 after popping the
only item left on the stack.

Signed-off-by: Roman Khimov <roman@nspcc.ru>

pkg/vm/ref_counter_test.go

@@ -3,6 +3,7 @@ package vm
 import (
 	"testing"
 
+	"github.com/nspcc-dev/neo-go/pkg/vm/opcode"
 	"github.com/nspcc-dev/neo-go/pkg/vm/stackitem"
 	"github.com/stretchr/testify/require"
 )
@@ -46,6 +47,19 @@ func TestRefCounter_Add(t *testing.T) {
 	require.Equal(t, 2, int(*r))
 }
 
+func TestRefCounterPopItem(t *testing.T) {
+	prog := makeProgram(opcode.POPITEM)
+	v := load(prog)
+	v.estack.PushVal(stackitem.NewArray([]stackitem.Item{stackitem.Make(42)}))
+	require.Equal(t, 2, int(v.refs))
+	runVM(t, v)
+	require.Equal(t, 1, v.estack.Len())
+	require.Equal(t, 1, int(v.refs))
+	_ = v.estack.Pop()
+	require.Equal(t, 0, v.estack.Len())
+	require.Equal(t, 0, int(v.refs))
+}
+
 func BenchmarkRefCounter_Add(b *testing.B) {
 	a := stackitem.NewArray(nil)
 	rc := newRefCounter()

pkg/vm/vm.go

@@ -1459,7 +1459,6 @@ func (v *VM) execute(ctx *Context, op opcode.Opcode, parameter []byte) (err erro
 		case *stackitem.Struct:
 			item.Remove(index)
 		}
-		v.refs.Remove(elem)
 
 	case opcode.SIZE:
 		elem := v.estack.Pop()