rr: coherent check for optional user part of advertised rr

(cherry picked from commit 48c42830f8d9f5927453dc091765c2434845c725)
(cherry picked from commit 01452997058ae07ee27f5e5e152a0f72d80a1dd8)

modules/rr/record.c

@@ -674,7 +674,7 @@ static inline int build_advertised_rr(struct lump* _l, struct lump* _l2, str *_d
 	memcpy(p, rr_prefix, rr_prefix_len);
 	p += rr_prefix_len;
 
-	if (user->len) {
+	if (user && user->len) {
 		memcpy(p, user->s, user->len);
 		p += user->len;
 		*p = '@';