{tcp|ipc}_connecter: check whether socket is valid before closing it

Calling close () when the underlying socket is invalid
triggers an assertion failure.

src/tcp_connecter.cpp

@@ -167,7 +167,8 @@ void zmq::tcp_connecter_t::start_connecting ()
 
     //  Handle any other error condition by eventual reconnect.
     else {
-        close ();
+        if (s != retired_fd)
+            close ();
         add_reconnect_timer ();
     }
 }