Fix an hang when max depth is reached

In this case SF stop searching and goes sleeping
waiting for a stop / ponderhit before to return
best move. So when a "stop" arrives we need to wake
up the main thread again.

Another regression introduced by 3aa471f2a9,
hopefully the last one.

Thanks to Otello1984 to reporting this.

No functional change.

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

src/thread.cpp

@@ -419,6 +419,7 @@ void ThreadsManager::wait_for_search_finished() {
 
   Thread* main = threads[0];
   lock_grab(main->sleepLock);
+  cond_signal(main->sleepCond); // In case is waiting for stop or ponderhit
   while (!main->do_sleep) cond_wait(sleepCond, main->sleepLock);
   lock_release(main->sleepLock);
 }