4f873376 as part of PR #3460 changed ffmpeg_mux_packet to
fail = !ffmpeg_mux_packet. ffe4c855 only reverted that one line
instead of the entire previous commit. When the change was
reintroduced in 6071098a as part of PR #3740, it became
fail = ffmpeg_mux_packet without the negation.
This commit reverts db1e6aa and fixes the logic check.
17b3873578