query / no aggregate of subquery
 The optimizer counts the aggregate functions that 
 appear as top level expressions (in all_fields) in 
 the current subquery. Later it makes a list of these
 that it uses to actually execute the aggregates in
 end_send_group().
 That count is used in several places as a flag whether
 there are aggregates functions.
 While collecting the above info it must not consider
 aggregates that are not aggregated in the current 
 context. It must treat them as normal expressions 
 instead. Not doing that leads to incorrect data about
 the query, e.g. running a query that actually has no
 aggregate functions as if it has some (and hence is
 expected to return only one row).
 Fixed by ignoring the aggregates that are not aggregated
 in the current context. 
 One other smaller omission discovered and fixed in the 
 process : the place of aggregation was not calculated for
 user defined functions. Fixed by calling 
 Item_sum::init_sum_func_check() and 
 Item_sum::check_sum_func() as it's done for the rest of 
 the aggregate functions.


mysql-test/r/subselect.result:
  Bug #27333: test case
mysql-test/t/subselect.test:
  Bug #27333: test case
sql/item_subselect.cc:
  Bug#27333: need select_lex to filter out
   aggregates that are not aggregated in
   the current select.
sql/item_sum.cc:
  Bug#27333: need select_lex to filter out
   aggregates that are not aggregated in
   the current select.
sql/item_sum.h:
  Bug#27333: calculate the place of 
   aggregation for user defined functions.
sql/sql_select.cc:
  Bug#27333: When counting the aggregated functions
   and collecting a list of them we must not consider
   the aggregates that are not aggregated in the local
   context as "local" : i.e. we must treat them as 
   normal functions and not add them to the aggregate
   functions list.
sql/sql_select.h:
  Bug#27333: need select_lex to filter out
   aggregates that are not aggregated in
   the current select.