ostbuild: Don't crash if there's no current revision either
ostbuild: Don't crash if there's no current revision either

For example if the user set local:

vrutkovs/vrutkovs.github.io,vrutkovs/vrutkovs.github.io,aperezdc/gnome-ostree,GNOME/gnome-ostree,chergert/gnome-continuous,GNOME/gnome-ostree,djdeath/gnome-ostree,djdeath/gnome-ostree,chergert/gnome-continuous,chergert/gnome-continuous,GNOME/gnome-ostree,vrutkovs/vrutkovs.github.io,aperezdc/gnome-ostree,djdeath/gnome-ostree,aperezdc/gnome-ostree