vm: add some checks for OVER and PICK, add OVER tests

These actually behaved correctly because of deferred sigsegv recover, but I
think it would be nice to print something more meaningful for users.

pkg/vm/vm.go

@@ -339,7 +339,13 @@ 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)
+		if a == nil {
+			panic("no second element found")
+		}
 		v.estack.Push(b)
 		v.estack.Push(a)
 
@@ -349,6 +355,9 @@ func (v *VM) execute(ctx *Context, op Instruction) {
 			panic("negative stack item returned")
 		}
 		a := v.estack.Peek(n)
+		if a == nil {
+			panic("no nth element found")
+		}
 		v.estack.Push(a)
 
 	case ROLL:

pkg/vm/vm_test.go

@@ -339,6 +339,35 @@ func TestXTUCKgood(t *testing.T) {
 	assert.Equal(t, int64(topelement), vm.estack.Peek(xtuckdepth).BigInt().Int64())
 }
 
+func TestOVERbadNoitem(t *testing.T) {
+	prog := makeProgram(OVER)
+	vm := load(prog)
+	vm.estack.PushVal(1)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestOVERbadNoitems(t *testing.T) {
+	prog := makeProgram(OVER)
+	vm := load(prog)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestOVERgood(t *testing.T) {
+	prog := makeProgram(OVER)
+	vm := load(prog)
+	vm.estack.PushVal(42)
+	vm.estack.PushVal(34)
+	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
+	assert.Equal(t, int64(42), vm.estack.Peek(0).BigInt().Int64())
+	assert.Equal(t, int64(34), vm.estack.Peek(1).BigInt().Int64())
+	assert.Equal(t, int64(42), vm.estack.Peek(2).BigInt().Int64())
+	assert.Equal(t, 3, vm.estack.Len())
+}
+
+
 func makeProgram(opcodes ...Instruction) []byte {
 	prog := make([]byte, len(opcodes)+1) // RET
 	for i := 0; i < len(opcodes); i++ {