refactoring: using an auxiliary function for diagonal lines

src/main/c/GameTic_Tac_Toe/game.c

@@ -32,8 +32,11 @@ char checkWinner() {
         if (verticalWinner != ' ') return verticalWinner;
     }
      // Überprüfe diagonale Linien
-    if (board[0][0] == board[1][1] && board[1][1] == board[2][2]) return board[0][0];
-    else if (board[0][2] == board[1][1] && board[1][1] == board[2][0]) return board[0][2];
+    char diagonal1Winner = checkLine(board[0][0], board[1][1], board[2][2]);
+    char diagonal2Winner = checkLine(board[0][2], board[1][1], board[2][0]);
+
+    if (diagonal1Winner != ' ') return diagonal1Winner;
+    if (diagonal2Winner != ' ') return diagonal2Winner;
 
  // Kein Gewinner
     return ' ';