Fix a race in idle_loop() exiting

When pondering threads are put to sleep, but when thinking
the threads are parked in idle_loop in a tight polling loop
checking for workIsWaiting falg.

So before we set the slave's flag workIsWaiting we have to
guarantee that all the slave data is already setup because
slave can start in any moment from there.

Rearrange the last loop to fix this race.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>