repo: Avoid leaking checksum_str for each download
repo: Avoid leaking checksum_str for each download

Closes: #232
Approved by: cgwalters

hughsie/libhif,Conan-Kudo/libhif,edynox/libdnf,rpm-software-management/libhif,rpm-software-management/libdnf,hughsie/libhif,hughsie/libhif,edynox/libdnf,hughsie/libhif,Conan-Kudo/libhif,rpm-software-management/libdnf,Conan-Kudo/libhif,rpm-software-management/libhif,edynox/libdnf,Conan-Kudo/libhif,rpm-software-management/libhif,rpm-software-management/libhif,edynox/libdnf,rpm-software-management/libdnf,rpm-software-management/libdnf