Sometimes next piece can't be seen

Hello,

Amazing bash scripting!
It has a bug that next piece can't be seen oftentimes.

Maybe it's related with get_random_next() function.
- Original source code
```
((next_piece_color = RANDOM % ${#colors[@]}
```
- Suggestion
```
((next_piece_color = ${colors[RANDOM % ${#colors[@]}]}))
```