avfilter/af_aecho: silence "maybe uninitialized warning"

if channels is 0 it actually would be uninitialized, thus an assert with comment is added

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/af_compand: silence "maybe uninitialized" warnings

if channels is 0 it actually would be uninitialized, thus an assert with comment is added

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>