Previously, the IIR filter wasn't clearing its previous Q interpolation, causing it to inherit the step factor from an older filter state (after calling e.g. fluid_iir_filter_reset()). This may have caused messing around with the Q, potentially becoming negative, and therefore causing a bunch of inf and nans to be generated whenever Q was passed to log() or sqrt(). This in turn would lead to very high audio gain output. It is not quite clear to me though, why this affected only some audio drivers, when either reverb or chorus was active, and why most notably the file renderer was completely unimpressed by this issue.

This PR properly clears the Q interpolation upon filter startup. Additionally, it was found that calling fluid_sec2tc with zero would have also raised a SIGFPE. This has been fixed as well.

Fixes #1464