Setting SESSION.GTID_NEXT=default right after setting
SESSION.GTID_NEXT='ANONYMOUS' causes an error
'ER_CANT_SET_GTID_NEXT_WHEN_OWNING_GTID', but this
changed gtid_next type from 'ANONYMOUS' to 'AUTOMATIC'.
So that the next transaction causes the above assertion error.

Setting SESSION.GTID_NEXT=default should not change the
gtid_next type if it causes an error.

@@sql/sys_vars.h
Removing the thd->variables.gtid_next.set_automatic() in
session_save_default(...), since it is too early to
set gtid_next type to 'AUTOMATIC' before causing a possible
error 'ER_CANT_SET_GTID_NEXT_WHEN_OWNING_GTID' and set_gtid_next(...)
has set gtid_next type to 'AUTOMATIC' after the error.