Fixes a regression introduced in 32c59a11,
becoming effective in 912969a3.

Fixes trimmed output of
ffmpeg -f lavfi -i "sine=d=0.01" -f lavfi -i "sine=d=1" -filter_complex "[0:a]anull[a1];[1:a]anull[a2]" -map "[a1]" -f null none -map "[a2]" -f framecrc -
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Marton Balint <cus@passwd.hu>