avfilter/af_headphone: Avoid intermediate buffers I

avfilter/af_headphone: Avoid intermediate buffers I

The headphone filter has two modes; in one of them (say A), it needs
certain buffers to store data. But it allocated them in both modes.
Furthermore when in mode A it also allocated intermediate buffers of the
same size, initialized them, copied their contents into the permanent
buffers and freed them.

This commit changes this: The permanent buffer is only allocated when
needed; the temporary buffer has been completely avoided.

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