Fix pool_send_frontend_exits() so that it checks the connection to backend if it actually exists. This is needed, for example, in the final stae of online recovery. pgpool child holds 1 connection to a backend. After failback one more backend added but there's no connection to the backend.
Fix pool_send_frontend_exits() so that it checks the connection to
backend if it actually exists. This is needed, for example, in the
final stae of online recovery. pgpool child holds 1 connection to
a backend. After failback one more backend added but there's no
connection to the backend.

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