Avoid "Cannot read property 'state' of undefined" error

When loading a set of scripts like the following:
```javascript
head.load([{'test1': 'http://localhost/js/test1.js'}]);
head.ready(['test1'], function(){
  head.load([{'test2': 'http://localhost/js/test2.js'}]);
});
head.ready(['test1','test2'], function(){
  head.load([{'test3': 'http://localhost/js/test3.js'}]);
});
```
an `Uncaught TypeError: Cannot read property 'state' of undefined` error is thrown and the `test3` script is not loaded.

Checking for `items[name]` on line 182 of the loader seems to clear this up.