Fixed peer hanging when receiving a bad synchronisation token

AlethZero seems to irrevocably hang when peer receives network data with an improper synchronisation token.

How to see it on linux:

* Run AlethZero
* Enable network
* Run `top` or equivalent on a terminal
* Run `dd if=/dev/zero bs=9 count=1 | nc localhost 30303` and watch AlethZero spike to 100% CPU and hang.

***

PS: not knowing how the peer should behave in this case, I assumed it should "gracefully disconnect" like it tries to do with an invalid message.