Add error propagation because packageset_to_pylist can return NULL (RhBug:1610456)
Add error propagation because packageset_to_pylist can return NULL (RhBug:1610456)

When SIGINT happens in packageset_to_pylist additional calls into
the Python/C API return NULL/-1 we have to check for that here
in order to avoid segfault.

hughsie/libhif,rpm-software-management/libdnf,rpm-software-management/libdnf,rpm-software-management/libhif,rpm-software-management/libdnf,hughsie/libhif,rpm-software-management/libhif,hughsie/libhif,rpm-software-management/libhif,hughsie/libhif,rpm-software-management/libhif,rpm-software-management/libdnf