Fixed crashes when using node-pdfinfo  

If for a strange reason the `results` variable is `undefined` or `null` in the `map` function the code will break because of the for loop used as bellow:
```js
// will break because of calling 'length' property on undefined
for(var i = 0; i < results.length ; i++){ 
   var result = results[i];
   // stuff
}
```
A good way to avoid these issues is to use the other for syntax:
```js
// will not break if results is undefined
for(var i in results){ 
  var result = results[i]; 
  // stuff 
}