The bug occurred when a subquery
- has a reference to outside, to grand-parent query or further up
- is converted to a semi-join (i.e. merged into its parent).

Then the reference to outside had form Item_ref(Item_field(...)).
- Conversion to semi-join would call item->fix_after_pullout() for the
  outside reference.
- Item_ref::fix_after_pullout would call Item_field->fix_after_pullout
- The Item_field would construct a new Name_resolution_context object
  This process ignored the fact that the Item_field does not belong to
  any of the subselects being flattened.
The result was crash in the next call to Item_field::fix_fields(), where
we would try to use an invalid Name_resolution_context object.

Fixed by not creating Name_resolution_context object if the Item_field's
context does not belong to the subselect(s) that were flattened.