Assertion `0' failed

If ROW item is a part of an expression that also has
aggregate function calls (COUNT/SUM/AVG...), a
"splitting" with an Item::split_sum_func2 function
is applied to that ROW item.
Current implementation of Item::split_sum_func2
replaces this Item_row with a newly created
Item_aggregate_ref reference to it.
Then the row cache tries to work with the
Item_aggregate_ref object as with the Item_row object:
row cache calls row-emulation methods such as cols and
element_index. Item_aggregate_ref (like it's parent
Item_ref) inherits dummy implementations of those
methods from the hierarchy root Item, and call to
them leads to failed assertions and wrong data
output.

Row-emulation virtual functions (cols, element_index, addr,
check_cols, null_inside and bring_value) of Item_ref have
been overloaded to forward calls to an underlying item
reference.




mysql-test/r/row.result:
  Added test case for bug #34620.
mysql-test/t/row.test:
  Added test case for bug #34620.
sql/item.h:
  Fixed bug #34620.
  Row-emulation virtual functions (cols, element_index, addr,
  check_cols, null_inside and bring_value) of Item_ref have
  been overloaded to forward calls to an underlying item
  reference.