Fix small bug in move_attacks_square()

Fix small bug in move_attacks_square()

We test if the piece moved in 'to' attacks the square 's' with:

bit_is_set(attacks_from(piece, to), s))

But we should instead consider the new occupancy, changed after
the piece is moved, and so test with:

bit_is_set(attacks_from(piece, to, occ), s))

Otherwise we can miss some cases, for instance a queen in b1 that
moves in c1 is not detected to attack a1 while instead she does.

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