Fix segmentation fault of child that occurs when startup packet has

Fix segmentation fault of child that occurs when startup packet has
no PostgreSQL user information.

When a startup packet has no PostgreSQL user specified, pgpool-II
terminated abnormally. You can reproduce it by

 $ psql -p 9999 -U ''

If enable_pool_hba is on, a child process terminates by segmentation
fault. Otherwise if enable_pool_hba is off, the error message is

 ERROR: pool_discard_cp: cannot get connection pool for user (null) database (null)

In both cases, psql terminates with no message on frontend.

To resolve it, if PostgreSQL user is not specified in startup packet,
the message as following is output to both log and frontend. This is
the same behavior as PostgreSQL.

 FATAL: no PostgreSQL user name specified in startup packet