journald: check whether sscanf has changed the value corresponding to %n

journald: check whether sscanf has changed the value corresponding to %n

It's possible for sscanf to receive strings containing all three fields
and not matching the template at the same time. When this happens the
value of k doesn't change, which basically means that process_audit_string
tries to access memory randomly. Sometimes it works and sometimes it doesn't :-)

See also https://bugzilla.redhat.com/show_bug.cgi?id=1059314.