Tested with zchunked rawhide repo and everything works correctly. If you want to test that it fixes the segfault, run:
dnf --enablerepo=rawhide list updates; rm /var/cache/dnf/rawhide*.solv*; /var/cache/dnf/rawhide-*/repodata/repomd.xml; dnf --enablerepo=rawhide list updates
The second time you run dnf, the Rawhide repo should be about 242KB instead of the usual 55MB, but, most importantly, you shouldn't get a segfault.
b09fe797cdbb4c8ff1e43a4be64bbaac1d1c5818 (Fix use after free)
d26a039f9f5b817f8bebddc7aa4f3f5a7a20cc69 (Fix uninitialized variable)
625335db13d62b80a5ab8cd374ce9d7c091d6e32 (Fix dereference before NULL)
62733a8606afa2139908577d3970cb04f1b16170 (Fix dereference before NULL and add extra checks)
af3d18b4fc492a83fdc47a4f980d9ab7c1990c0c (Set umask before running mkstemp because some UNIX flavors don't do this)
Ok, changing my karma to +1 because the problem lies in a F27->F28 upgrade. A new install doesn't see this problem.
To fix the bug if you've done a F27->F28 update, change all the files in /var/log/nginx to have owner.group of root.root.