Ensure we do not try to send data to null servers

Primus will crash if it get's a null server back.

An easy way to trigger this is with [omega-supreme](https://github.com/primus/omega-supreme) and [metroplex](https://github.com/primus/metroplex) :
```javascript
primus.forward.spark("a-disconnected-spark-id", {some: "data"})
```

This can happen when two clients are sending messages directly to the other spark. Right now the server crashes. Michael Bay would say the server looks something like this:

![Server crash](http://how2becool.com/wp-content/uploads/2014/06/transformers_2_explosion_2.jpg)