When precision is fixed and volume is 0, filter_frame does not
perform any operation on the output buffer. This works if the
output buffer has been allocated and zeroed with ff_get_audio_buffer
but not if the input buffer is used as output buffer.

Fix this by not using the input buffer as output buffer if
precision is fixed and volume is 0.
Signed-off-by: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>