PGM subscription forwarding fixed

PGM when using in XPUB socket has to subscribe for all the messages
as it has no idea what the subscribers are interesred in.
This generic subscribe message was malformed. Fixed.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

src/pgm_sender.cpp

@@ -94,7 +94,8 @@ void zmq::pgm_sender_t::plug (io_thread_t *io_thread_, session_t *session_)
     //  what messages are peers interested in. Because of that we have to
     //  subscribe for all the messages.
     msg_t msg;
-    msg.init ();
+    msg.init_size (1);
+    *(unsigned char*) msg.data () = 1;
     bool ok = session_->write (&msg);
     zmq_assert (ok);
     session_->flush ();