MDEV-4475: Replication from MariaDB 10.0 to 5.5 does not work

The problem was the Gtid_list event which is logged to the binlog in
10.0 and is not understood by the 5.5 server.

This event is supposed to be replaced with a dummy event for 5.5
servers. But the very first event logged in the very first binlog
has an empty list of GTID, which makes the event too short to be
replacable with an empty event.

The fix is to pad the empty Gtid_list event to be big enough to
be replacable by a dummy event.

mysql-test/suite/innodb/r/group_commit_binlog_pos.result

@@ -31,6 +31,6 @@ a
 1
 2
 3
-InnoDB: Last MySQL binlog file position 0 920, file name ./master-bin.000001
+InnoDB: Last MySQL binlog file position 0 922, file name ./master-bin.000001
 SET DEBUG_SYNC= 'RESET';
 DROP TABLE t1;