Audio example: Channel adjust in song view can get out of valid range
With `examples/audio.nim` in nico v0.3.2 on OSX:
Using pcA+pcUp / pcA+pcDown to adjust the channel in the song view can cause the channel to get out of the valid range [0..64]. This seems to be due to incorrect boundary checks in `audio.nim gameUpdate(126-130)`.

Resulting stack trace:
```
~/nico/examples/audio.nim(345) audio
~/.nimble/pkgs/nico-0.3.2/nico.nim(2935) run
~/.nimble/pkgs/nico-0.3.2/nico/backends/sdl2.nim(1662) run
~/.nimble/pkgs/nico-0.3.2/nico/backends/sdl2.nim(1272) step
~/.nimble/pkgs/nico-0.3.2/nico/backends/sdl2.nim(1519) queueMixerAudio
~/nico/examples/audio.nim(40) musicUpdate
~/.choosenim/toolchains/nim-1.4.2/lib/system/fatal.nim(49) sysFatal
Error: unhandled exception: index 240 not in 0 .. 63 [IndexDefect]
Error: execution of an external program failed: '~/nico/examples/audio '
```