Fix re-run issue when job size is set to "unknown"

Fix re-run issue when job size is set to "unknown"

Assert that td->o.size is valid before comparing the IO amount
we did to it. This prevents re-running a job because we think
there's more work to do. But if we haven't set ->o.size to a
valid value, then that must not determine whether to re-run
the loop or not.

Signed-off-by: Jens Axboe <axboe@kernel.dk>