Add file distance condition in move_is_legal()

Found another missed control in move_is_legal() thanks to
brute force testing.

No functional change.

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

src/position.cpp

@@ -714,6 +714,10 @@ bool Position::move_is_legal(const Move m, Bitboard pinned) const {
       // piece (en passant captures was handled earlier).
       if (color_of_piece_on(to) != them)
           return false;
+
+      // From and to files must be one file apart, avoids a7h5
+      if (abs(square_file(from) - square_file(to)) != 1)
+          return false;
       break;
 
       case DELTA_N: