lib/zonecut: check for parent of root

lib/zonecut.c

@@ -254,12 +254,12 @@ int kr_zonecut_find_cached(struct kr_context *ctx, struct kr_zonecut *cut, const
 
 	/* Start at QNAME parent. */
 	name = knot_wire_next_label(name, NULL);
-	while (txn) {
+	while (txn && name) {
 		if (fetch_ns(ctx, cut, name, txn, timestamp) == 0) {
 			update_cut_name(cut, name);
 			return kr_ok();
 		}
-		if (name[0] == '\0') {
+		if (!name || !name[0]) {
 			break;
 		}
 		/* Subtract label from QNAME. */