Fix saving animated gifs with giflib 5.x
Fix saving animated gifs with giflib 5.x

The APP extension was not correctly saved, since its blocks were
ignored (only the leader and the trailer was written).

500px/magick,500px/magick,zuriu/magick,rainycape/magick,rainycape/magick,rainycape/magick,zuriu/magick,500px/magick,zuriu/magick