Fix FEN validation of pawns on first or eighth rank

The FEN validation check intended to reject pawns on the first or eighth rank uses the `Rank` enum values in a bitwise OR operation:

`if (pieces(PAWN) & (RANK_1 | RANK_8))`

`RANK_1 | RANK_8` evaluates to the integer `0 | 7 == 7` instead of a bitboard, so the expression only tests squares A1, B1 and C1. As a result, unsupported positions with pawns elsewhere on the first or eighth rank are silently accepted. For instance, `position fen 3P3k/8/8/8/8/8/8/3K4 w - - 0 1` is accepted even though the pawn on d8 makes the position unsupported.

Use the `Rank1BB | Rank8BB` bitboard constants so any pawn on the first or eighth rank are correctly rejected.

closes https://github.com/official-stockfish/Stockfish/pull/6887

No functional change

AUTHORS

@@ -198,6 +198,7 @@ Nour Berakdar (Nonlinear)
 Ofek Shochat (OfekShochat, ghostway)
 Ondrej Mosnáček (WOnder93)
 Ondřej Mišina (AndrovT)
+Onur Zungur (zungur)
 Oskar Werkelin Ahlin
 Ömer Faruk Tutkun (OmerFarukTutkun)
 Pablo Vazquez

src/position.cpp

@@ -270,7 +270,7 @@ Position::set(const string& fenStr, bool isChess960, StateInfo* si) {
     if (rank != RANK_1 || file != FILE_NB)
         return PositionSetError("Invalid FEN. Board state encoding ended but cursor not at end.");
 
-    if (pieces(PAWN) & (RANK_1 | RANK_8))
+    if (pieces(PAWN) & (Rank1BB | Rank8BB))
         return PositionSetError("Unsupported position. Pawns on the first or eighth rank.");
 
     if (count<KING>(WHITE) != 1 || count<KING>(BLACK) != 1)