Fix uninitialized variable in error case in pool_do_auth().
Fix uninitialized variable in error case in pool_do_auth().

If there's no valid backend, pgpool will return garbage pid to
frontend in auth phase. Actually because no backend is available,
frontend will be disconnected later on. So this is not harmless.
Per Coverity report "1127331 Uninitialized scalar variable".

treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres