Fix crash in read-only mode when ontologies.gdvb is missing

We shouldn't unref the manager->ontologies object if loading the
new ontologies failed. Fixes a crash in the fallback code path.
2 jobs for sam/survive-missing-ontologies-gvdb in 2 minutes and 3 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #234317
test-fedora-latest

00:02:03

failed #234315
test-fedora-latest

00:01:50