trx_rseg_update_wsrep_checkpoint() writes excessive amount of redo log

The function trx_rseg_update_wsrep_checkpoint() was modified as part of the Galera 4 merge. The following code is causing warning in GCC 8.2:

	if (must_clear_rsegs) {

		XID null_xid;

		memset(&null_xid, 0, sizeof null_xid);

		null_xid.null();

Also, it is writing unnecessarily much redo log. 10.4 introduced the MLOG_MEMSET record, which should be used here. We should need that object null_xid at all.