Don't poll the connection monitor if there are no connections added.

This caused the backends to return an error code, which
in turn ended the mainloop. However, several other things
also might occur in the main loop, such as DNS lookups which
come prior to creating any connections that in turn would be monitored.

src/network/backend.c

@@ -132,9 +132,10 @@ struct timeout_queue* net_backend_get_timeout_queue()
  */
 int net_backend_process()
 {
-	int res;
+	int res = 0;
 	size_t secs = timeout_queue_get_next_timeout(&g_backend->timeout_queue, g_backend->now);
 
+	if (g_backend->common.num)
 		res = g_backend->handler.backend_poll(g_backend->data, secs * 1000);
 
 	g_backend->now = time(0);