If a set function with a outer reference s(outer_ref) cannot be aggregated 
the outer query against which the reference has been resolved then MySQL
interpretes s(outer_ref) in the same way as it would interpret s(const).
Hovever the standard requires throwing an error in this situation.
Added some code to support this requirement in ansi mode.
Corrected another minor bug in Item_sum::check_sum_func.
 


mysql-test/r/subselect.result:
  Added a test case for bug #27348.
mysql-test/t/subselect.test:
  Added a test case for bug #27348.
sql/item_sum.cc:
  Fixed bug #27348.
  If a set function with a outer reference s(outer_ref) cannot be aggregated 
  the outer query against which the reference has been resolved then MySQL
  interprets s(outer_ref) in the same way as it would interpret s(const).
  Hovever the standard requires throwing an error in this situation.
  Added some code to support this requirement in ansi mode.
  Corrected another minor bug in Item_sum::check_sum_func.