use enum retired_fd instead of -1

using -1 causes a warning on Windows platform because SOCKET is unsigned.

src/socket_base.cpp

@@ -121,7 +121,7 @@ zmq::socket_base_t *zmq::socket_base_t::create (int type_, class ctx_t *parent_,
             errno = EINVAL;
             return NULL;
     }
-    if (s->mailbox.get_fd () == -1)
+    if (s->mailbox.get_fd () == retired_fd)
         return NULL;
 
     alloc_assert (s);