postprocess: Don't g_critical if previous commit doesn't have version
postprocess: Don't g_critical if previous commit doesn't have version

I sometimes run "rpm-ostree compose tree" directly, mainly so I can
use gdb and/or nonstandard options.  In this case I don't get
version numbers injected.

That happens to trigger a bug in this code.

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