MFH: r542123

multimedia/ffmpeg: update to 4.3.1

Changes: https://git.ffmpeg.org/gitweb/ffmpeg.git/blob/n4.3.1:/Changelog
ABI: https://abi-laboratory.pro/tracker/timeline/ffmpeg/
Reported by: Repology
Security: CVE-2020-13904 CVE-2020-14212
Approved by: ports-secteam blanket