Fix brekage from previous patches

It is interesting the fact that we need to test for
move_is_castle(m) anyway and not relying on testing
if destination square is attacked. Indeed the latter
condition fails if the castling rook is attacked,
castling is coded as "king captures the rook" but it
is legal in that case.

Verified no functional change with beginning of the series.

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