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

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

src/ipc_connecter.cpp

@@ -154,6 +154,7 @@ void zmq::ipc_connecter_t::start_connecting ()
 
     //  Handle any other error condition by eventual reconnect.
     else {
+        if (s != retired_fd)
             close ();
         add_reconnect_timer ();
     }

src/tcp_connecter.cpp

@@ -167,6 +167,7 @@ void zmq::tcp_connecter_t::start_connecting ()
 
     //  Handle any other error condition by eventual reconnect.
     else {
+        if (s != retired_fd)
             close ();
         add_reconnect_timer ();
     }