Disable sending messages on SUB socket

The ability was inherited from XSUB socket.
Now it's properly disabled.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

src/sub.cpp

@@ -51,7 +51,7 @@ int zmq::sub_t::xsetsockopt (int option_, const void *optval_,
 
     //  Pass it further on in the stack.
     int err = 0;
-    int rc = xsend (&msg, 0);
+    int rc = xsub_t::xsend (&msg, 0);
     if (rc != 0)
         err = errno;
     zmq_msg_close (&msg);
@@ -59,3 +59,16 @@ int zmq::sub_t::xsetsockopt (int option_, const void *optval_,
         errno = err;
     return rc;
 }
+
+int zmq::sub_t::xsend (zmq_msg_t *msg_, int options_)
+{
+    //  Overload the XSUB's send.
+    errno = ENOTSUP;
+    return -1;
+}
+
+bool zmq::sub_t::xhas_out ()
+{
+    //  Overload the XSUB's send.
+    return false;
+}

src/sub.hpp

@@ -35,6 +35,8 @@ namespace zmq
     protected:
 
         int xsetsockopt (int option_, const void *optval_, size_t optvallen_);
+        int xsend (zmq_msg_t *msg_, int options_);
+	bool xhas_out ();
 
     private: