Constant crackling in EngineFilterButterworth8Band::process()

This is a fix for https://bugs.launchpad.net/mixxx/+bug/1326001

Below you can see the performance differences.

_My notebook (with RT Scheduling disabled):_
Without the patch:
Stat("_processBandpass()","count=2.10739e+06,sum=1.89391e+09ns,average=898.699ns,min=461ns,max=422348ns,variance=1.01698e+06ns^2,stddev=1008.45ns")

With the patch:
Stat("_processBandpass()",count=2.61939e+06,sum=2.50817e+09ns,average=957.538ns,min=477ns,max=663356ns,variance=1.04328e+06ns^2,stddev=1021.41ns")

---

_Asus eeePC with RT Scheduling enabled:_
Without the patch:
Stat("_processBandpass()","count=3.43757e+06,sum=8.52755e+09ns,average=2480.69ns,min=1467ns,max=138215ns,variance=1.43556e+06ns^2,stddev=1198.15ns")

With the patch:
Stat("_processBandpass()","count=1.85549e+06,sum=4.71517e+09ns,average=2541.2ns,min=1536ns,max=131022ns,variance=1.88703e+06ns^2,stddev=1373.69ns")


I'll add NaN checks in all filters if it is necessary.