ITS #9418 slapd-bconfig: check against potential nullptr deref

Deduced from AC checking above `ce->ce_parent` is potentially null so
it's better to check again before dereference.
3 jobs for !208 with bugfix/slapd-check-against-potential-nullptr-deref in 32 minutes and 39 seconds (queued for 34 minutes and 39 seconds)
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