Fix killing of threads that haven't started

Fix killing of threads that haven't started

If there is a thread that has not started yet (for instance with a long
startdelay or a stonewall) then a SIGINT will fire off terminate_threads
which runs through all of the threads and calls kill(pid, SIGTERM).
However the threads that have not started yet have a pid of 0.  When you
call kill(0, SIGTERM) it kills the entire thread group which can extend
out of fio.  I have included a short patch which fixes this for me.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>