Use correct occupancy in connected_threat()

When checking if a move defends the threatened piece
we correctly remove from the occupancy bitboard the
moved piece. This patch removes from the occupancy also
the threatening piece so to consider the cases of moves
that defend the threatened piece x-raying through the
threat move.

As example in this position:
r3k2r/p1ppqp2/Bn4p1/3p1n2/4P1N1/5Q1P/PPP2P1P/R3K2R w KQkq - 1 10

The threat black move is dxe4. With this patch we include
(and so don't prune) white's Bb7 that would be pruned otherwise.

The number of affected position is very low, around 1% of cases,
so we don't expect ELO changes, neverthless this is the logical
and natural thing to do.

Patch suggested by Hongzhicheng.

new bench: 5323798

src/search.cpp

@@ -1426,8 +1426,8 @@ split_point_start: // At split points actual search starts from here
         && (   PieceValue[MG][pos.piece_on(tfrom)] >= PieceValue[MG][pos.piece_on(tto)]
             || type_of(pos.piece_on(tfrom)) == KING))
     {
-        // Update occupancy as if the piece is moving
-        Bitboard occ = pos.pieces() ^ mfrom ^ mto;
+        // Update occupancy as if the piece and the threat are moving
+        Bitboard occ = pos.pieces() ^ mfrom ^ mto ^ tfrom;
         Piece piece = pos.piece_on(mfrom);
 
         // The moved piece attacks the square 'tto' ?