Fix broken voice mixing (#582)
Fix broken voice mixing (#582)

This fixes a regression introduced in 907ec27a9ee3cb22217d61e045ec36ae164e3ee0

When rendering a voice, there are 3 cases to consider: silent, playing,
and finished. When optimizing away the memset, I incorrectly assumed that
a voice cannot switch between playing and silence (like crazy) while
rendering FLUID_MIXER_MAX_BUFFERS_DEFAULT. Apparently this does not
hold true, esp. when rendering at sample rates ~96kHz.
FluidSynth/fluidsynth,FluidSynth/fluidsynth,FluidSynth/fluidsynth,FluidSynth/fluidsynth