avfilter/af_headphone: Only keep one AVFrame at a time

avfilter/af_headphone: Only keep one AVFrame at a time

Despite the headphone filter only using one AVFrame at a time, it kept
an array each of whose entries contained a pointer to an AVFrame at all
times; the pointers were mostly NULL. This commit instead replaces them
by using a single pointer to an AVFrame on the stack of the only
function that actually uses them.

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