fix divide by zero in zmq::lb_t::sendpipe

This is based on https://github.com/zeromq/zeromq3-x/pull/116/files

src/lb.cpp

@@ -137,7 +137,9 @@ int zmq::lb_t::sendpipe (msg_t *msg_, pipe_t **pipe_)
     more = msg_->flags () & msg_t::more? true: false;
     if (!more) {
         pipes [current]->flush ();
-        current = (current + 1) % active;
+
+        if (++current >= active)
+            current = 0;
     }
 
     //  Detach the message from the data buffer.