If pseudo_thread_id is set to 0 (or implicitly set to 0 because
it was attempted to be set to a negative value), then the
thread_id attribute of the Gtid_log_event will not be written
at all, whereas the Query_log_event still will write
thread_id=0.  The underlying problem was that the
FL_EXTRA_THREAD_ID would only be added if thread_id was greater
than 0.

This patch fixes this by changing the condition to mark the
FL_EXTRA_THREAD_ID when generating a GTID log event, such that
it will always be written on the primary, and if on the slave,
the rpl_group_info's gtid_ev_flags_extra is checked to see if
the flag was present when reading it in.

Reviewed By:
============
Andrei Elkin <andrei.elkin@mariadb.com>