"update existingtable set anycolumn=nonexisting order by nonexisting" would crash
the server.

Though we would find the reference to a field, that doesn't mean we can then use
it to set some values.  It could be a reference to another field.  If it is NULL, 
don't try to use it to set values in the Item_field and instead return an error.

Over the previous patch, this signals an error at the location of the error, rather
than letting the subsequent deref signal it.


mysql-test/r/order_by.result:
  Verify that all permutations work.
mysql-test/t/order_by.test:
  Verify that all permutations work.
sql/item.cc:
  When the field is NULL, don't dereference it when we set_field().  
  Instead, raise an error.