Follow-up to MDEV-13570 Assertion failure !srv_read_only_mode in --innodb-read-only shutdown when buf_resize_thread is active

logs_empty_and_mark_files_at_shutdown(): Actually skip the debug assertion
when the buf_resize_thread is active. The previous fix skipped the
debug assertion failure when buf_dump_thread is active. Both these
threads are created also in innodb_read_only mode. Depending on how
fast these threads react to the shutdown signal, the debug assertion
could be triggered.

There is no impact on non-debug servers, and very little impact on
debug servers either, because in innodb_read_only shutdown, no InnoDB
files will need to be written.