Add invariant check for delegset in rctx_nextserver()

The get_nameservers path in rctx_nextserver() is only reachable from
rctx_referral(), which already detaches fctx->delegset.  Assert that
it is NULL rather than redundantly detaching it, since
dns_view_bestzonecut() requires *delegsetp == NULL.

lib/dns/resolver.c

@@ -9379,6 +9379,7 @@ rctx_nextserver(respctx_t *rctx, dns_message_t *message,
 		} else {
 			name = fctx->domain;
 		}
+		INSIST(fctx->delegset == NULL);
 		result = dns_view_bestzonecut(fctx->res->view, name, fname,
 					      dcname, fctx->now, findoptions,
 					      true, true, &fctx->delegset);