journalctl: find boot ID more gracefully in corrupted journal

journalctl: find boot ID more gracefully in corrupted journal

In discover_next_boot(), first we find a new boot ID based on the value
stored in the entry object. Then, find the tail (or head when we are going
upwards) entry of the boot based on the _BOOT_ID= field data.

If boot IDs of an entry in the entry object and _BOOT_ID field data
are inconsistent, which may happen on corrupted journal, then previously
discover_next_boot() failed with -ENODATA.

This makes the function check if the two boot IDs in each entry are
consistent, and skip the entry if not.

Fixes the failure of `journalctl -b -1` for 'truncated' journal:
https://github.com/systemd/systemd/pull/29334#issuecomment-1736567951