fix: if parallel is run with a sync requestor results in null exception

If you run RQ.parallel with synchronous requestor then you get a null exception. 

Example:
```
var syncRequestor = function(requesition) {
	requesition(10);
};

RQ.parallel([
	syncRequestor
])(function (success, failure) {

});
```

Since the requestor is synchronous , it means that the finish function is called before line 392. The finish function sets the quashes array to null, hence the null exception. 

This pull request fixes this problem.