ffmpeg: Fix CVE-2020-35964, CVE-2020-35965

Backport the CVE patches from upstream:
https://github.com/FFmpeg/FFmpeg/commit/27a99e2c7d450fef15594671eef4465c8a166bd7
https://github.com/FFmpeg/FFmpeg/commit/3e5959b3457f7f1856d997261e6ac672bba49e8b



CVE:
CVE-2020-35964
CVE-2020-35965

Signed-off-by: Khairul Rohaizzat Jamaluddin <khairul.rohaizzat.jamaluddin@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>