vm: limit types accepted for PICKITEM, fix #965

bafdb916a0 change was wrong (probably brought
from neo-vm 3.0 at the state at which it existed back then), neo-vm 2.x
doesn't allow PICKITEM for arbitrary types.

pkg/vm/vm.go

@@ -1029,12 +1029,7 @@ func (v *VM) execute(ctx *Context, op opcode.Opcode, parameter []byte) (err erro
 			}
 			v.estack.Push(&Element{value: t.value[index].Value.Dup()})
 		default:
-			arr := obj.Bytes()
-			if index < 0 || index >= len(arr) {
-				panic("PICKITEM: invalid index")
-			}
-			item := arr[index]
-			v.estack.PushVal(int(item))
+			panic("PICKITEM: unknown type")
 		}
 
 	case opcode.SETITEM:

pkg/vm/vm_test.go

@@ -1367,9 +1367,7 @@ func TestPICKITEMByteArray(t *testing.T) {
 	vm := load(prog)
 	vm.estack.PushVal([]byte{1, 2})
 	vm.estack.PushVal(1)
-	runVM(t, vm)
-	assert.Equal(t, 1, vm.estack.Len())
-	assert.Equal(t, makeStackItem(2), vm.estack.Pop().value)
+	checkVMFailed(t, vm)
 }
 
 func TestPICKITEMDupArray(t *testing.T) {