Fixups for GCC's unused variable warning.

This patch keeps DSPFilters from triggering GCC's ```-Wunused-but-set-variable``` warning.

Merging both this and pull request 10 (https://github.com/vinniefalco/DSPFilters/pull/10) allows DSPFilters to build without triggering any of the GCC warnings enabled by ```-Wall```:

    $ CXXFLAGS='-Wall' ./waf

This allows users to build with ```-Wall -Werror``` ("all" warnings enabled, treat warnings as errors), which is common enough practice to be worth supporting.