nameserver: add sanity assert to get_next_closer()

src/knot/nameserver/nsec_proofs.c

@@ -120,6 +120,7 @@ static const knot_dname_t *get_next_closer(const knot_dname_t *closest_encloser,
 	// make name only one label longer than closest_encloser
 	ssize_t ce_labels = knot_dname_labels(closest_encloser, NULL);
 	ssize_t qname_labels = knot_dname_labels(name, NULL);
+	assert(qname_labels > ce_labels);
 	for (int i = 0; i < (qname_labels - ce_labels - 1); ++i) {
 		name = knot_dname_next_label(name);
 	}