Actual fix for multiple filter sets in AudioFilterBiquad

My manipulation of the contents of *dest instead of the value of dest
itself was foolish at best - forgot to leave the * off for my intent.

the version of the example in this commit compiled and appeared to do the same as the previous version using the updated filter_biquad.cpp but please check that you agree before accepting pull request - just using my ears and nothing else to confirm same attenuation is applied by both versions at home.