Fixed #1: The game stops working when the board is full

[liplum]

After inspecting the code, I found this function was the cause. https://github.com/angjelkom/flutter_2048/blob/ca1c660f5e737ee51fb4259130b08c7677c62433/lib/managers/board.dart#L140-L148 The loop is endless when the indexes gets all 16 numbers. And based on this algorithm, the probability of generating tiles at each location is not.