Fix for CONC-600: Do not release rpl_event if event_type is unknown

[007gzs]

When i got a unsupport event_typ like PREVIOUS_GTIDS_LOG_EVENT, mariadb_rpl_fetch will free rpl_event and return null;
If my code is

while ((event = mariadb_rpl_fetch(rpl, event)))
{
	//todo
}

when i got unsupport event, the loop will finish.

I can check rpl->buffer_size out of function mariadb_rpl_fetch like

	while (1)
	{
		while ((event = mariadb_rpl_fetch(rpl, event)))
		{
			//todo
		}
		if (rpl->buffer_size == 0) break;
		unsupport_event_type = rpl->buffer[5];
               //todo
	}

but rpl_event->memroot will be ma_free_root and ma_init_alloc_root in next call mariadb_rpl_fetch

[9EOR9]

Thanks for you contribution (applied rev. 1e2f6d5).