Fix to the position function

Some elements of the array of results passed by the function ```search``` to its callback can be undefined. This isn't handled by the function ```position``` which don't check it before trying to access to its properties. It can be seen easily with the next simple script:


```javascript
var google = require("google-tools");

var options = {
  url:"https://github.com",
  q:"SOME QUERY ABOUT SOMETHING",
  num:100,
  start:0
};

google.position(options,function(err, r){
  //Some callback
});
```

What returns the error:

```Cannot call method 'substr' of undefined```

It can be avoided checking the element of the array before acceding its properties. This implementation could not fix the root problem but would regain the functionality, which is currently broken.