Fix for Bug#8801: the bug was in co-operation of Item_ref

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.


sql/item.cc:
  Set member Item_ref::ref to null whenever the item itself is substituted 
  with another item.
  This is necessary if we want to re-execute a prepared statement next time.
  
  Additionally Item_ref::fix_fields() implementation was cleaned up
  (by Monty and myself) to reduce the number of if branches. This
  doesn't change the logic of this function.