Fruit spawning when there is no place left

[Glutexo]

When there are no free coordinates in the field, it is unable for a new piece of fruit to spawn. The game halts in an infinite loop.

In this case, a new piece of fruit shall not appear. The field is probably completely filled by the snake’s body and the game will be over in the next turn.

For this to be possible, a more clever fruit spawning mechanism is needed. See #1.