Fix GH-16389: Assertion failure in ext/ldap/ldap.c:2718

[nielsdos]

@Girgias IIRC the reference support is only in master right, and so this should target master too?

[Girgias]

Thank for catching this, I did indeed forget to deref this here :|

I do wonder if we shouldn't "merge" the checking and the "actually do stuff" loops together, like it is done in other cases.

But that is a follow-up PR anyway.

[nielsdos]

I do wonder if we shouldn't "merge" the checking and the "actually do stuff" loops together, like it is done in other cases.

Likely, but this isn't too important I feel like and indeed a follow-up anyway