Callback parameter needs an extra check

My test code looks like this:
```js
var evaluate = require('static-eval');
var parse = require('esprima').parse;

var src = 'foo(function (obj) { return obj.x })';
var ast = parse(src).body[0].expression;
var result = evaluate(ast, {
foo: function (func) {
return func({x: 1})
},
});

console.log(result);
```

When I run it, I get:
```
$ node test.js
/home/stathis/src/jellyfish-jellyscript/node_modules/static-eval/index.js:134
return obj[node.property.name];
^

TypeError: Cannot read property 'x' of null
at walk (/home/stathis/src/jellyfish-jellyscript/node_modules/static-eval/index.js:134:27)
at walk (/home/stathis/src/jellyfish-jellyscript/node_modules/static-eval/index.js:152:20)
at walk (/home/stathis/src/jellyfish-jellyscript/node_modules/static-eval/index.js:171:20)
at walk (/home/stathis/src/jellyfish-jellyscript/node_modules/static-eval/index.js:112:25)
at module.exports (/home/stathis/src/jellyfish-jellyscript/node_modules/static-eval/index.js:204:7)
at Object. (/home/stathis/src/jellyfish-jellyscript/test.js:6:14)
at Module._compile (internal/modules/cjs/loader.js:1068:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:1097:10)
at Module.load (internal/modules/cjs/loader.js:933:32)
at Function.Module._load (internal/modules/cjs/loader.js:774:14)
```

whereas when I change
```js
var src = 'foo(function (obj) { return obj.x })';`
```
to
```js
var src = 'foo(function (obj) { return obj && obj.x })';`
```
I get a successful result:
```
$ node test.js
1
```

This looks like a bug to me, as my callback's input here is just `{x: 1}`, why would I need to make sure `obj` exists?