Fix the warning : points to uninitialised byte(s) for evthr_pool_defer

evthr_res
        evthr_defer(evthr_t * thread, evthr_cb cb, void * arg) {
            int cur_backlog;
            evthr_cmd_t cmd;

            /*Add this line*/
            memset(&cmd, '\0', sizeof (cmd));

![error-ethr](https://f.cloud.github.com/assets/154762/803346/7805d5fa-edee-11e2-8300-2f77defd87b2.png)