passwd: Avoid EINVAL if previous passwd data is empty somehow
passwd: Avoid EINVAL if previous passwd data is empty somehow

Maybe we should make this into an explicit error, but anyways
I somehow ended up with an empty /usr/etc/passwd in the tree
contents, I think due to bugs in earlier work there.

This causes fmemopen to return EINVAL, which errors out the
compose.  Let's stumble forwards here.

Now that I think about it, it might be a valid case to have an
existant but empty /usr/etc/passwd in the tree - when we migrate to
systemd-sysusers, I think we'll want an empty file there by default.

mdshuai/rpm-ostree,giuseppe/rpm-ostree,mdshuai/rpm-ostree,giuseppe/rpm-ostree,tmp-user/rpm-ostree,giuseppe/rpm-ostree,tmp-user/rpm-ostree,tmp-user/rpm-ostree,mdshuai/rpm-ostree,tmp-user/rpm-ostree,giuseppe/rpm-ostree,mdshuai/rpm-ostree