OptionDef.u is only an offset (i.e. its off member) iff OPT_FLAG_OFFSET
is true. Otherwise, the pointer arithmetic can be undefined behaviour.
UBSan warns about this (on 32bit arches):
src/fftools/ffmpeg_opt.c:102:15: runtime error: pointer index expression with base 0xffa4db10 overflowed to 0x56059a50

This commit fixes this by checking for OPT_FLAG_OFFSET first.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>