Always mark socket as retired when closing ipc_listener

When closing an ipc listener, the library may try to unlink
the associated file. When this fails, the underlying
socket is not marked as retired and this triggers
assertion failure.

Fixes issue #397

src/ipc_listener.cpp

@@ -171,6 +171,8 @@ int zmq::ipc_listener_t::close ()
     int rc = ::close (s);
     errno_assert (rc == 0);
 
+    s = retired_fd;
+
     //  If there's an underlying UNIX domain socket, get rid of the file it
     //  is associated with.
     if (has_file && !filename.empty ()) {
@@ -182,7 +184,6 @@ int zmq::ipc_listener_t::close ()
     }
 
     socket->monitor_event (ZMQ_EVENT_CLOSED, endpoint.c_str(), s);
-    s = retired_fd;
     return 0;
 }