sd-journal: always fallback to find entry by realtime

sd-journal: always fallback to find entry by realtime

Consider the following situation:
- There are two journal files (x and y), that contains entries for two boots (X and Y).
- The journal file x contains entries of the boot X, and y contains
  entries of Y.
- Nevertheless x does not contains entries of boot Y, it contains
  the _BOOT_ID= data object of boot Y. Of course, the data object is not
  referenced by any entries in the journal file x.

In such situation, when the current location of sd_journal is the head
of journal y, that is, the first entry of the boot Y,
sd_journal_previous() failed without this change, and
'journalctl --boot -NUM' for boot X failed.

Fixes #29275.