In particular, avoid to leave around the seg->avf pointer to freed
structure, and fix crash with:
ffmpeg -f lavfi -i testsrc -c:v h264 -map 0 -f segment foo-%d.ts
(cherry picked from commit 169065fb)

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