Support castling move with NAG in FenParser.

[MPolleke]

Hi, loading a PGN with castling move with a NAG results in a hard crash on PHP 8.0.2. This PR should fix that.

PHP Error:
Fatal error: Uncaught Error: Unsupported operand types: string + int
in chessParser/FenParser0x88.php on line 1253

Sample code:

$pgn = '[Event "chessParser test"] [Site "test: Castling with NAG"] [Date "2021.02.16"] [Round "1"] [White "White player"] [Black "Black player"] [Result "*"] [PlyCount "12"] 1. e4 e5 2. d3 d6 3. Nc3 Nc6 4. Bd2 Bd7 5. Qe2 Qe7 6. O-O-O O-O-O= *';
$pgnparser = new PgnParser();
$pgnparser->setPgnContent($pgn);
$game = $pgnparser->getFirstGame();