fails after the first time
  
Two separate problems : 
  1. When flattening joins the linked list used for name resolution 
  (next_name_resolution_table) was not updated.
  Fixed by updating the pointers when extending the table list
  
  2. The items created by expanding a * (star) as a column reference
  were marked as fixed, but no cached table was assigned to them 
  (unlike what Item_field::fix_fields does).
  Fixed by assigning a cached table (so the re-preparation is done
  faster).
  
Note that the fix for #2 hides the fix for #1 in most cases
(except when a table reference cannot be cached).