multimedia/ffmpeg: update to 4.0.3

Changes: https://git.ffmpeg.org/gitweb/ffmpeg.git/blob/n4.0.3:/Changelog
ABI: https://abi-laboratory.pro/tracker/timeline/ffmpeg/
PR: 227726 (for tracking)
Security: CVE-2018-15822
MFH: 2018Q4 (bug and fuzz fixes)