Fix two bugs

1. Fix the bug that made black move first in a new game when the old
   game was ended during blacks turn.
2. Fix bug that offered the promotion dialog to the player when a pawn
   was moved on the last rank from any square

Also, a late initializer error was fixed because the wrong move variable
was used when a pawn reached the last rank.

lib/chess/chess_square.dart

@@ -83,8 +83,12 @@ class ChessSquare extends StatelessWidget {
         }
       },
       builder: (context, candidateData, rejectedData) {
-        var maxDrags =
+        var maxDrags = kDebugMode
-            kDebugMode ? 1 : (ChessBloc.turnColor == ChessBloc.myColor ? 1 : 0);
+            ? 1
+            : ((ChessBloc.turnColor == ChessBloc.myColor) &&
+                    (containedPiece?.color == ChessBloc.turnColor)
+                ? 1
+                : 0);
 
         return Container(
           color: color,

lib/chess_bloc/chess_bloc.dart

@@ -36,6 +36,7 @@ class ChessBloc extends Bloc<ChessEvent, ChessBoardState> {
   }
 
   void initBoard(InitBoard event, Emitter<ChessBoardState> emit) {
+    turnColor = ChessColor.white;
     ChessPosition.getInstance().resetToStartingPosition();
     emit(ChessBoardState(ChessColor.white, ChessColor.white,
         ChessPosition.getInstance().currentPosition));

lib/chess_bloc/promotion_bloc.dart

@@ -16,6 +16,14 @@ class PromotionBloc extends Bloc<PromotionEvent, PromotionState> {
     PawnMovedToPromotionField event,
     Emitter<PromotionState> emit,
   ) {
+    switch (event.colorMoved) {
+      case ChessColor.white:
+        if (event.move.from.row != 7) return;
+        break;
+      case ChessColor.black:
+        if (event.move.from.row != 2) return;
+        break;
+    }
     move = event.move;
     emit(PromotionState(
         showPromotionDialog: true, colorMoved: event.colorMoved));