lslogins: use sd_journal_get_data() in proper way

lslogins: use sd_journal_get_data() in proper way

man sd_journal_get_data:
    The returned data is in a read-only memory map and is only valid until the next invocation
    of sd_journal_get_data().

It means that use data after 3x sd_journal_get_data() is really bad
idea. It also seems better to not assume the fields are zero
terminated as journal API works with void* and size_t to address the
data.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1984704
Signed-off-by: Karel Zak <kzak@redhat.com>