Functions over sum functions wasn't set up correctly for the ORDER BY clause
which leads to a wrong order of the result set.

The split_sum_func() function is called now for each ORDER BY item that
contains a sum function to set it up correctly.


mysql-test/t/order_by.test:
  Added a test case for bug#25376: Incomplete setup of ORDER BY clause results in a wrong result.
mysql-test/r/order_by.result:
  Added a test case for bug#25376: Incomplete setup of ORDER BY clause results in a wrong result.
sql/sql_select.cc:
  Bug#25376: Incomplete setup of ORDER BY clause results in a wrong result.
  The split_sum_func() function is called now for each ORDER BY item that
  contains a sum function to set it up correctly.