lfe-filter: Deal with empty input chunks

lfe-filter: Deal with empty input chunks

It is possible that we get a zero-length memchunk to work with.
Specifically, this happens the resampler (which is called before the
lfe-filter) consumes all the input data, but does not (yet) produce any
output data.

Reproduced using:

  pulseaudio --resample-method=soxr-mq
  pactl load-module module-null-sink sink_name=lfe_test channels=3 channel_map=front-left,front-right,lfe
  paplay --raw /dev/zero --rate=48000 -d lfe_test

Thanks to the original reporter for the backtrace:

Bug: https://bugs.launchpad.net/ubuntu/+source/pulseaudio/+bug/1496577