The generic code should be able to finish the streams just fine initializing
and flushing the filters and codecs properly.

Fixes the following command:

ffmpeg -f lavfi -i "testsrc=d=0.1[out0];aevalsrc=0:d=0[out1]" -af apad -shortest -f framecrc -
Signed-off-by: Marton Balint <cus@passwd.hu>