Now check if an en-passent capture is possible before setting the board 'EPSq' entry. This is mainly to fix a subtle 'bug' in computing the hash of positions where there is a potential en-passent capture.

The following position is an example: 8/6bb/8/8/R1pP2k1/4P3/P7/K7 b - d3 0 1. Here black cannot take the d4 pawn en-passent because it would leave him in check (note that this is a rather unusual case of discovered check as two pieces have 'moved'), accordingly it should be considered the same (from the search's point of view) as the same position but where white's last move was not d2d4 (e.g. d3d4 instead, this position 8/6bb/8/8/R1pP2k1/4P3/P7/K7 b - - 0 1). If we want the search to consider them equal, we must make the hash keys compute equal. Hence this fix.