avfilter/af_headphone: Don't allocate unused element in array

avfilter/af_headphone: Don't allocate unused element in array

The headphone filter uses an array with as many elements as the
filter has inputs to store some per-input information; yet actually it
only stores information for all inputs except the very first one (which
is special for this filter). Therefore this commit modifies the code to
remove this unused element.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>