nss_db: protect against empty mappings

[Florian Weimer]

* DJ Delorie:

> Andreas Schwab <schwab@suse.de> writes:
>> If calling is_selinux_enabled has side effects then it cannot be elided.
>> If it doesn't, then it is pointless.  Assignment to a write-only
>> variable doesn't change that.
>
> Sorry, by "side effects" I meant "the program has a runtime dependency
> on selinux.so".  The program is never run so it doesn't matter what the
> function *does*.  The code is intentionally overcomplicated to ensure
> gcc doesn't optimize away the call, either now or in the future.

Just print the return value, then?

Thanks,
Florian