pgsql: Check if the role exists before doing more complex ident and

Log Message:
-----------
Check if the role exists before doing more complex ident and Kerberos
authentication checks in the backend.

Gavin Sherry

Modified Files:
--------------
    pgsql/src/backend/libpq:
        auth.c (r1.147 -> r1.148)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/auth.c.diff?r1=1.147&r2=1.148)
        hba.c (r1.158 -> r1.159)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/hba.c.diff?r1=1.158&r2=1.159)