vm: do not pop items in OVER

pkg/vm/vm.go

@@ -408,15 +408,10 @@ func (v *VM) execute(ctx *Context, op Instruction) {
 		}
 
 	case OVER:
-		b := v.estack.Pop()
-		if b == nil {
-			panic("no top-level element found")
-		}
-		a := v.estack.Peek(0)
+		a := v.estack.Peek(1)
 		if a == nil {
 			panic("no second element found")
 		}
-		v.estack.Push(b)
 		v.estack.Push(a)
 
 	case PICK:

pkg/vm/vm_test.go

@@ -920,6 +920,8 @@ func TestOVERbadNoitem(t *testing.T) {
 	vm.estack.PushVal(1)
 	vm.Run()
 	assert.Equal(t, true, vm.state.HasFlag(faultState))
+	assert.Equal(t, 1, vm.estack.Len())
+	assert.Equal(t, makeStackItem(1), vm.estack.Pop().value)
 }
 
 func TestOVERbadNoitems(t *testing.T) {