sd-journal: check sd-event state before setting up post change timer

sd-journal: check sd-event state before setting up post change timer

The similar check already exists in schedule_post_change().

The function is currently called at two places.
- journal_file_open() in sd-journal:
  In this case, if the timer is not set up, then journal_file_post_change()
  will be called at the end of journal_file_append_entry(). So, the necessary
  task will be done sequentially when an journal entry is stored to the opened
  journal file. That is desired when the function is called at outside of the
  event loop.
- server_open_journal() in journald:
  This is not called after we exit the event loop.

So, we can safely do nothing in the function if the event loop is being
finished or already finished.

Fixes #30644.