MDEV-20625: MariaDB asserting when enabling wsrep=on

mysql-test/suite/wsrep/r/MDEV-20625.result

+SET GLOBAL wsrep_on=ON;
+SHOW GLOBAL STATUS LIKE 'wsrep_cluster_size';
+Variable_name	Value
+wsrep_cluster_size	1

mysql-test/suite/wsrep/t/MDEV-20625.cnf

+!include ../my.cnf
+
+[mysqld.1]
+wsrep-on=OFF
+binlog-format=ROW
+wsrep-provider=@ENV.WSREP_PROVIDER
+wsrep-cluster-address='gcomm://'
+innodb_autoinc_lock_mode=2

mysql-test/suite/wsrep/t/MDEV-20625.test

+#
+# Check SHOW GLOBAL STATUS after dynamic setting WSREP=ON
+#
+--source include/have_innodb.inc
+--source include/have_wsrep_provider.inc
+--source include/have_binlog_format_row.inc
+
+SET GLOBAL wsrep_on=ON;
+SHOW GLOBAL STATUS LIKE 'wsrep_cluster_size';