[core] Do not SEGV if no statitics collected.

core/stat.c

@@ -108,6 +108,9 @@ stat_print(struct tbuf *buf)
 void
 stat_age(ev_timer *timer, int events __unused__)
 {
+	if (stats == NULL)
+		return;
+
 	for (int i = 0; i <= stats_max; i++) {
 		if (stats[i].name == NULL)
 			continue;