Rebinding on reconnection to allow for the pipe blocking will set the event sink…

Rebinding on reconnection to allow for the pipe blocking will set the event sink again, which will cause an assert to be triggered. I have modified that to allow for setting the same sink to be OK. I believe this should be ok - on termination, if a pipe hasn't reconnected, it would just attempt to be erased from the pipes list again, which is reasonable

src/pipe.cpp

@@ -83,7 +83,7 @@ void zmq::pipe_t::set_peer (pipe_t *peer_)
 void zmq::pipe_t::set_event_sink (i_pipe_events *sink_)
 {
     // Sink can be set once only.
-    zmq_assert (!sink);
+    zmq_assert (!sink || sink_ == sink);
     sink = sink_;
 }