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 3aa471f2a9cb,
hopefully the last one.

Thanks to Otello1984 to reporting this.

No functional change.

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