https://bugs.openldap.org/show_bug.cgi?id=10029
--- Comment #2 from gray@nxg.name gray@nxg.name --- I don't do this deliberately (I don't think it's a sensible configuration any more than you do). I ran into this when, as part of a test harness, I invoked slapd from a Make variant which 'helpfully' sets this limit to unlimited. (I think that counts as a defect in that program, and have reported it as such, but...).
So I agree with you that having this unlimited is not sensible. But it's a possible configuration, and it would seem useful for slapd to at least do a sanity check here, and if it finds an insane value to do something more useful than simply exit with an assertion violation.
Even exiting with an error would be something, as you suggest. Alternatively, calling setrlimit at this point, with an arbitrary large-ish value like 65535 would restore the process to a reasonable configuration, and be more-or-less a one-line change.