Fix regression: engine hangs while pondering

Fix regression: engine hangs while pondering

After a "stop" due to a ponder miss Xboard sends
immediately the new position to search, without
waiting for engine to effectively stop the search.

It is not clear if this is a GUI bug (as I suspect)
or allowed behaviour, but because it won't be fixed
anyway workaround this issue making listener thread
to switch to in-sync mode as soon as a "stop" command
is received.

Thanks to Mike Whiteley for reporting this.

No functional change.

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