- when create_ref_for_key() is constructing a ref access for
  a table that's inside a SJ-Materialization nest, it may not 
  use references to fields of tables that are unside the nest (because 
  these fields will not yet have values when ref access will be used)
  
  The check was performed in the first of create_ref_for_key's loops (the 
  one which counts how many key parts are usable) but not in the second
  (the one which actually fills the TABLE_REF structure).