Created attachment 574330 [details, diff]
After rebuilding media-video/ffmpeg-3.4.5 with a newer toolchain I got segfaults for most videos I tried to play (with ffplay or mplayer). I found the following upstream commit that addresses the issue but is missing from the 3.4 branch:
The attached patch based on that commit (changes to version.h disabled because they don't apply) fixes the issue for me.
The (unpatched) code writes to some arrays which are declared "const", so it's no surprise this causes segfaults. I think this should be fixed in the stable version even though it apparently works by chance with some toolchains and CFLAGS.