init: Fix initialisation crash

Fix a case where initialisation fails without setting
the passed-in GError and the caller assumes it will be
set, and add a guard to catch the condition in case it
happens again in the future.

Part-of: <gstreamer/gst-editing-services!218>
29 jobs for !218 with backport-mr-217-into-1.18-init-crash in 30 minutes and 55 seconds (queued for 9 seconds)
merge request