Fix to bug #673, http://bugs.skysql.com/show_bug.cgi?id=673

dcb.c:dcb_hashtable_stats:division by zero
hashtable.c:hashtable_get_stats: NULL-pointer reference
service.c:serviceStartPort:set service->users NULL to avoid referring to freed memory
users.c:dcb_usersPrintf: NULL-pointer reference
debugcmd.c:convert_arg: changed return value to 1 in case of error, 0 (==NULL) is valid but it indicates that there are no users loaded.
	execute_cmd: fixed command handling

server/core/service.c

@@ -217,6 +217,7 @@ GWPROTOCOL	*funcs;
 					service->name)));
 				hashtable_free(service->users->data);
 				free(service->users);
+				service->users = NULL;
 				dcb_free(port->listener);
 				port->listener = NULL;
 				goto retblock;