audio/claxon: remove some panic points
audio/claxon: remove some panic points

This remove some possible panic points due to unwrapping and
underflowing integer maths.

Signed-off-by: Luca BRUNO <d11e81b0438fe9a6fbb85b72e5bb4c36a65f49c7@lucabruno.net>

sdroege/rsplugin,sdroege/rsplugin