journal-oops: use the length result of sd_journal_get_data()

journald doesn't guarantee NULL terminated strings returned from
sd_journal_get_data(). It usually works but not always.

This patch fixes the issue by using the length of field data instead of
assuming that string is NULL terminated.

Resolves: #1141549

Signed-off-by: Jakub Filak <jfilak@redhat.com>