v9_10_sub: decrement_reference() not effective for node cleanup

decrement_reference() doesn't appear effective for node cleanup in 9.10-sub. This is because clean_cache_node() fails to set node->data to NULL after freeing the rbtdb_data_t when all data under it has been cleaned up.

Something like this is required:

diff --git a/lib/dns/rbtdb.c b/lib/dns/rbtdb.c
index 5730d3b14e..eb23137681 100644
--- a/lib/dns/rbtdb.c
+++ b/lib/dns/rbtdb.c
@@ -1990,6 +1990,13 @@ clean_cache_node(dns_rbtdb_t *rbtdb, dns_rbtnode_t *node) {
                                           rbtdb->common.mctx,
                                           clean_iptree_nodedata,
                                           rbtdb);
+
+               if ((data->nonecs_data == NULL) &&
+                   (data->ecs_root == NULL))
+               {
+                       isc_mem_put(rbtdb->common.mctx, data, sizeof(*data));
+                       node->data = NULL;
+               }
        }
 
        node->dirty = 0;