Fix a shutdown bug in update_rpz_cb()

When shutting down, the cleanup path should not try to destroy
'newnodes', because it is NULL at that point.

Introduce another label for the "shuttingdown" scenario.

(cherry picked from commit 975d16230b)

lib/dns/rpz.c

@@ -1918,7 +1918,7 @@ update_rpz_cb(void *data) {
 
 	result = dns__rpz_shuttingdown(rpz->rpzs);
 	if (result != ISC_R_SUCCESS) {
-		goto cleanup;
+		goto shuttingdown;
 	}
 
 	isc_ht_init(&newnodes, rpz->rpzs->mctx, 1, ISC_HT_CASE_SENSITIVE);
@@ -1939,6 +1939,7 @@ update_rpz_cb(void *data) {
 cleanup:
 	isc_ht_destroy(&newnodes);
 
+shuttingdown:
 	rpz->updateresult = result;
 }