fd: only add valid fd to mainloop

src/lxc/mainloop.c

@@ -84,6 +84,8 @@ int lxc_mainloop_add_handler(struct lxc_epoll_descr *descr, int fd,
 	struct epoll_event ev;
 	struct mainloop_handler *handler;
 	struct lxc_list *item;
+	if (fd < 0)
+		return -1;
 
 	handler = malloc(sizeof(*handler));
 	if (!handler)