MDEV-22521 Server crashes in traverse_role_graph_up or Assertion `user' fails...

MDEV-22521 Server crashes in traverse_role_graph_up or Assertion `user' fails in traverse_role_graph_impl

mysql-test/suite/roles/drop_current_role.result

+create role r;
+set role r;
+drop role r;
+revoke all on *.* from current_role;
+ERROR 42000: There is no such grant defined for user 'r' on host ''

mysql-test/suite/roles/drop_current_role.test

+--source include/not_embedded.inc
+#
+# MDEV-22521 Server crashes in traverse_role_graph_up or Assertion `user' fails in traverse_role_graph_impl
+#
+create role r;
+set role r;
+drop role r;
+error ER_NONEXISTING_GRANT;
+revoke all on *.* from current_role;

sql/sql_acl.cc

@@ -4977,6 +4977,8 @@ static void propagate_role_grants(ACL_ROLE *role,
                                   enum PRIVS_TO_MERGE::what what,
                                   const char *db= 0, const char *name= 0)
 {
+  if (!role)
+    return;
 
   mysql_mutex_assert_owner(&acl_cache->lock);
   PRIVS_TO_MERGE data= { what, db, name };