Fill the seconds field of invalid timestamps

Fill the seconds field of invalid timestamps

We used to fill only nsec with -1. However, this means that the sec
field is kept uninitialized. This means that reading it would be
undefined behavior, which is bad. This doesn't seem to happen in
dlogutil as is, but it would be possible when choosing a timestamp to sort
on is be implemented.

Also, a slight change to sort vector is done so that it works well in
such situation.

Change-Id: I501adad093c0782433d014395e8cc752f7d48574