with view-merge algorithm and prepared statements: in case when some
Item_ref pointing to a view column was substituted with a reference 
pointing to the view expression for that column
Item_ref::ref member of the original Item_ref was left pointing to 
not_found_item (0x1).
As we currently perform expression substition part of the view-merge 
algorithm per each execution of a prepared statement or stored procedure, 
we need to preserve original Item_ref objects usable.