Fix failover() to deal with the case when no former primary node exists.

Fix failover() to deal with the case when no former primary node exists.

Consider a case when no primary node exists when Pgpool-II starts
up. In this case Req_info->primary_node_id is -1. failover() did not
consider this and skipped to call find_primary_node_repeatedly().
Also follow_master_command was not executed if
Req_info->primary_node_id is -1.

This commit fixes the bug above.

Discussion: https://www.pgpool.net/pipermail/pgpool-hackers/2022-February/004114.html