Item_sum_distinct::setup(THD*): Assertion

There was an assertion to detect a bug in ROLLUP
implementation. However the assertion is not true
when used in a subquery context with non-cacheable
statements.
Fixed by turning the assertion to accepted case
(just like it's done for the other aggregate functions). 


mysql-test/r/func_group.result:
  Bug #31156: test case
mysql-test/t/func_group.test:
  Bug #31156: test case
sql/item_sum.cc:
  Bug #31156: make it OK to call setup() several times:
   done for (e.g.) scalar subquery