support and test Eval() without an existing scope

otto.go

@@ -302,6 +302,11 @@ func (self Otto) Run(src interface{}) (Value, error) {
 // already defined in the current stack frame. This is most useful in, for
 // example, a debugger call.
 func (self Otto) Eval(src interface{}) (Value, error) {
+	if self.runtime.scope == nil {
+		self.runtime.enterGlobalScope()
+		defer self.runtime.leaveScope()
+	}
+
 	value, err := self.runtime.cmpl_eval(src)
 	if !value.safe() {
 		value = Value{}

otto_test.go

@@ -1441,6 +1441,22 @@ func TestOttoEval(t *testing.T) {
 
 		is(err, nil)
 	})
+
+	tt(t, func() {
+		vm := New()
+
+		_, err := vm.Eval("null")
+		is(err, nil)
+
+		vm.Set("a", 1)
+		vm.Set("b", 2)
+
+		v, err := vm.Eval("a + b")
+		is(err, nil)
+		r, err := v.Export()
+		is(err, nil)
+		is(r, 3)
+	})
 }
 
 func Test_objectLength(t *testing.T) {