Fix assertion in pgm_sender_t::plug() (LIBZMQ-303)

Opening any PGM socket gives this assertion. The problem is in
pgm_sender_t::plug() which is incorrectly testing the return value from
session::write().
Signed-off-by: Martin Lucina <martin@lucina.net>

src/pgm_sender.cpp

@@ -98,8 +98,8 @@ void zmq::pgm_sender_t::plug (io_thread_t *io_thread_, session_base_t *session_)
     msg_t msg;
     msg.init_size (1);
     *(unsigned char*) msg.data () = 1;
-    bool ok = session_->write (&msg);
-    zmq_assert (ok);
+    int rc = session_->write (&msg);
+    errno_assert (rc == 0);
     session_->flush ();
 }