CID 452571: Use after free in lib/isc/httpd.c

Coverity Scan claims use after free in lib/isc/httpd.c after #4011 (closed).

/lib/isc/httpd.c: 956 in isc_httpdmgr_shutdown()
950     	httpd = ISC_LIST_HEAD(httpdmgr->running);
951     	while (httpd != NULL) {
952     		if (httpd->handle != NULL) {
953     			httpd_request(httpd->handle, ISC_R_SUCCESS, NULL,
954     				      httpd);
955     		}
>>>     CID 452571:    (USE_AFTER_FREE)
>>>     Dereferencing freed pointer "httpd".
956     		httpd = ISC_LIST_NEXT(httpd, link);
957     	}
958     	UNLOCK(&httpdmgr->lock);
959     
960     	isc_nmsocket_close(&httpdmgr->sock);
961