avfilter/af_headphone: Avoid intermediate buffer III

avfilter/af_headphone: Avoid intermediate buffer III

The headphone filter allocates a pair of buffers to be used as
intermediate buffers lateron: Before every use they are zeroed, then
some elements of the buffer are set and lateron the complete buffers are
copied into another, bigger buffer. These intermediate buffers are
unnecessary as the data can be directly written into the bigger buffer.
Furthermore, the whole buffer has been zeroed initially and because no
piece of this buffer is set twice (due to the fact that duplicate
channel map entries are skipped), it is unnecessary to rezero the part
of the big buffer that is about to be written to.

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