Fix race while exiting

Fix again TimerThread::idle_loop() to prevent a
theoretical race with 'exit' flag in ~Thread().

Indeed in Thread d'tor we raise 'exit' and then
call notify() that is lock protected, so we
have to check again for 'exit' before going to
sleep in idle_loop().

Also same change in Thread::idle_loop() where we
now check for 'exit' before to go to sleep.

No functional change.

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

src/search.cpp

@@ -1588,7 +1588,7 @@ void Thread::idle_loop() {
           // particular we need to avoid a deadlock in case a master thread has,
           // in the meanwhile, allocated us and sent the wake_up() call before we
           // had the chance to grab the lock.
-          if (!is_searching && Threads.sleepWhileIdle)
+          if (!is_searching && !do_exit)
               sleepCondition.wait(mutex);
 
           mutex.unlock();