Use the correct tuplestore read pointer in a NamedTuplestoreScan.

Use the correct tuplestore read pointer in a NamedTuplestoreScan.

Tom Kazimiers reported that transition tables don't work correctly when
they are scanned by more than one executor node.  That's because commit
18ce3a4ab allocated separate read pointers for each executor node, as it
must, but failed to make them active at the appropriate times.  Repair.

Thomas Munro

Discussion: https://postgr.es/m/20180224034748.bixarv6632vbxgeb%40dewberry.localdomain