af_hrtf: initialize coefficient arrays

Sometimes, --af=hrtf produces heavy artifacts or silence. It's possible
that this commit fixes these issues. My theory is that usually, the
uninitialized coefficients quickly converge to sane values as more audio
is filtered, which would explain why there are often artifacts on init,
with normal playback after that. It's also possible that sometimes, the
uninitialized values were NaN or inf, so that the artifacts (or silence)
would never go away.

Fix this by initializing the coefficients to 0. I'm not sure if this is
correct, but certainly better than before.

See issue #1104.