Fix last search info carried over to mate position
When starting search in a mate or stalemate position, Stockfish does not
even care to reinitialize and start worker threads. However after search
all threads are checked for the best move.
This can lead to bestmove and info beeing carried over from the last
search.
Example session:
setoption name threads value 7
go movetime 4000
position startpos moves f2f3 e7e5 g2g4 d8h4
go movetime 4000