sox_flow_effects: only allocate buffers if needed
sox_flow_effects: only allocate buffers if needed

If there's no effect using two or more flows (because there is only
one channel, or because they are all MCHAN effects), interleave
buffers aren't needed. Don't allocate them in that case.

Motiejus/sox,Distrotech/sox,Distrotech/sox,Motiejus/sox,mhartzel/sox_personal_fork,MageSlayer/sox,Motiejus/sox,uklauer/sox,cbagwell/sox,mhartzel/sox_personal_fork,mhartzel/sox_personal_fork,pcqpcq/sox,davel/sox,mhartzel/sox_personal_fork,jacksonh/sox,jacksonh/sox,MageSlayer/sox,Distrotech/sox,CaptainHayashi/sox,Motiejus/sox,jacksonh/sox,MageSlayer/sox,pcqpcq/sox,uklauer/sox,cbagwell/sox,MageSlayer/sox,jacksonh/sox,Distrotech/sox,cbagwell/sox,davel/sox,uklauer/sox,pcqpcq/sox,MageSlayer/sox,davel/sox,uklauer/sox,CaptainHayashi/sox,davel/sox,pcqpcq/sox,CaptainHayashi/sox,cbagwell/sox,CaptainHayashi/sox