[CM] adding an extra channel

[Bill Schottstaedt]

> the LADSPA plugin requires 4 channels. So when I read a 3 channel 
> file, I need to create a fourth, empty (no signal) channel. 

Doesn't apply-ladspa take a list of readers?  Perhaps an easy fix would
be to allow #f (as a null-reader) in that list, then apply_ladspa (in C)
would send a stream of 0's in each channel that had no reader -- this
is a one line change, if it seems like the right thing.