Query_log_event::pack_info does not check flags2_inited

Recent MDEV-19964 patch did not check flags2_inited before
to access flags2 itself.
The latter may be randomly non-zero (see Query_log_event constructor) which
eventulizes on rpl.rpl_mariadb_slave_capability with the following mismatch pattern:

-slave-relay-bin.000005	#	Query	#	#	BEGIN

+slave-relay-bin.000005	#	Query	#	#	set foreign_key_checks=1, check_constraint_checks=1; BEGIN

where the '+' extra is cause by the random flags2.

The case should be fixed with following the pattern of first checking flags2_inited.