fix bad assertion

[This assertion](https://github.com/lynaghk/zmq-async/blob/69ff7682d38ed964d84a7471a3b51e5492067bc3/src/com/keminglabs/zmq_async/core.clj#L160) will fail under legitimate circumstances.

Consider the following sequence of events:

1. A pull socket is registered
2. Async loop has the socket in the pairings map
3. ZMQ loop has the socket in the socks map
4. The in channel of the socket is closed, and at the same time a message is received from the ZMQ socket
5. Async loop will dissoc the socket from the pairings map
6. ZMQ loop will relay the message to the async loop
7. Async loop will look up the socket from the pairings map, but the socket in question has already been dissoc'ed
8. An exception is thrown, after which nothing works anymore.