After a "stop" do not read new input until search finishes

Unfortunatly xboard sends immediately the new position to
search after sending "stop" when we have a ponder miss.

Becuase main thread position is not copied but is referenced
directly from root position and the latter is modified by
the "position.." UCI command we end up with the working position
that changes under our feet while the search is still recovering
after the "stop" and this causes a crash.

This happens only with the (broken) xboard, native UCI does not
have this problem.

Reported by otello1984

No functional change.

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