Revert "Remove unnecessary extra test in pipe assertion - now we are terminating pipes we don't need to allow the case in which a pipe is rebound to the same sink"

This reverts commit 29f8d9ca15.

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_;
 }