In FFmpeg before 4.2, avcodec_open2 in libavcodec/utils.c allows a NULL pointer dereference and possibly unspecified other impact when there is no valid close function pointer.
https://www.debian.org/security/2020/dsa-4722
https://usn.ubuntu.com/4431-1/
https://security.gentoo.org/glsa/202003-65
https://lists.debian.org/debian-lts-announce/2021/01/msg00026.html
https://github.com/FFmpeg/FFmpeg/commit/8df6884832ec413cf032dfaa45c23b1c7876670c