Segfault in git_config_set_multivar_in_file_gently with direct_io in FUSE filesystem

I am using git with a simple in-memory FUSE filesystem. When I enable
direct_io, I get a segfault from
git_config_set_multivar_in_file_gently during git clone.

I have full reproduction instructions using Go and macOS at
https://github.com/josharian/gitbug. It also includes a stack trace in
case anyone wants to try blind debugging. Happy to provide more info
if it will help.

Thanks,
Josh