Fixed abort when running in foreground but stdin is closed

A forgotten else-clause caused BIRD to treat some pseudo-random place in
memory as fd-pair. This was happening only on startup of the first
thread in group and the value there in memory was typically zero ... and
writing to stdin succeeded.

When running BIRD with stdin not present (like systemd does), it died on
this spurious write. Now it seems to work correctly.

Thanks to Daniel Suchy <danny@danysek.cz> for reporting.

http://trubka.network.cz/pipermail/bird-users/2023-May/016929.html