nss_db: protect against empty mappings

[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.

The only thing we ever do with that program is call ldd on it to
enumerate the DSOs that need to be installed into the testroot.