Use "#ifdef CATALOG_VARLEN" to protect nullable fields of pg_authid.

Use "#ifdef CATALOG_VARLEN" to protect nullable fields of pg_authid.

This gives a stronger guarantee than a mere comment against accessing these
fields as simple struct members.  Since rolpassword is in fact varlena,
it's not clear why these didn't get marked from the beginning, but let's
do it now.

Michael Paquier