sd-journal: do not read unnecessary object

sd-journal: do not read unnecessary object

In journal_file_next_entry(), if the passed offset matches an entry object,
then generic_array_bisect() returns the object, but the object we
requested is the next (or previous) object. Hence, we should not validate
the object returned by generic_array_bisect(), otherwise it may fail
when the journal is corrupted.

Note the validity of the entry object that should be returned by
journal_file_next_entry() will be checked in the following generic_array_get().
So, when journal_file_next_entry() succeeds, the returned object is
always validated.