Fix use-after-free around DISTINCT transition function calls.

Fix use-after-free around DISTINCT transition function calls.

Have tuplesort_gettupleslot() copy the contents of its current table slot
as needed. This is based on an approach taken by tuplestore_gettupleslot().
In the future, tuplesort_gettupleslot() may also be taught to avoid copying
the tuple where caller can determine that that is safe (the
tuplestore_gettupleslot() interface already offers this option to callers).

Patch by Peter Geoghegan. Fixes bug #14344, reported by Regina Obe.

Report: <20160929035538.20224.39628@wrigleys.postgresql.org>

Backpatch-through: 9.6