Fix sharing Agg transition state of DISTINCT or ordered aggs.

If a query contained two aggregates that could share the transition value,
we would correctly collect the input into a tuplesort only once, but
incorrectly run the transition function over the accumulated input twice,
in finalize_aggregates(). That caused a crash, when we tried to call
tuplesort_performsort() on an already-freed NULL tuplestore.

Backport to 9.6, where sharing of transition state and this bug were
introduced.

Analysis by Tom Lane.

Discussion: https://www.postgresql.org/message-id/ac5b0b69-744c-9114-6218-8300ac920e61@iki.fi