returns wrong results

Casting AVG() to DECIMAL led to incorrect results when the arguments
had a non-DECIMAL type, because in this case
Item_sum_avg::val_decimal() performed the division by the number of
arguments twice.

Fixed by changing Item_sum_avg::val_decimal() to not rely on
Item_sum_sum::val_decimal(), i.e. calculate sum and divide using
DECIMAL arithmetics for DECIMAL arguments, and utilize val_real() with
subsequent conversion to DECIMAL otherwise.


mysql-test/r/func_group.result:
  Added a test case for bug #34512.
mysql-test/t/func_group.test:
  Added a test case for bug #34512.
sql/item_sum.cc:
  Do not use Item_sum_sum::val_decimal() in Item_sum_avg::val_decimal()
  because the first one, depending on the arguments type, may return
  either the sum of the arguments, or the average calculated by the
  virtual val_real() method of Item_sum_avg. Instead, do our own
  calculation based on the arguments type.