construct references invalid name.
Derived tables currently cannot use outer references.
Thus there is no outer context for them.
The 4.1 code takes this fact into account while the 
Item_field::fix_outer_field code of 5.0 lost the check that blocks
any attempts to resolve names in outer context for derived tables.


mysql-test/r/union.result:
  Added a test case for bug #26661.
mysql-test/t/union.test:
  Added a test case for bug #26661.
sql/item.cc:
  Fixed bug #26661.
  Derived tables currently cannot use outer references.
  Thus there is no outer context for them.
  The 4.1 code takes this fact into account while the 
  Item_field::fix_outer_field code of 5.0 lost the check that blocks
  any attempts to resolve names in outer context for derived tables.