journalctl: fix --follow with non-matching filter

journalctl: fix --follow with non-matching filter

When there is no matching entry stored in journal, then initial call of
`sd_journal_previous()` following `sd_journal_seek_tail()` returns
zero, and does not move the read pointer.
In the main loop, on every journal event, we call `sd_journal_next()`,
even though the current location is tail, and it takes no effect.

In such a case, we need to call `sd_journal_previous()` instead of
`sd_journal_next()`.