Pass EXEC_FLAG_REWIND when initializing a tuplestore scan.

Since a rescan is possible, we must be able to rewind.

Thomas Munro, per a report from Prabhat Sabu

Discussion: http://postgr.es/m/CAEepm=2=Uv5fm=exqL+ygBxaO+-tgmC=o+63H4zYAXi9HtXf1w@mail.gmail.com

src/backend/executor/nodeNamedtuplestorescan.c

@@ -107,7 +107,7 @@ ExecInitNamedTuplestoreScan(NamedTuplestoreScan *node, EState *estate, int eflag
 	scanstate->relation = (Tuplestorestate *) enr->reldata;
 	scanstate->tupdesc = ENRMetadataGetTupDesc(&(enr->md));
 	scanstate->readptr =
-		tuplestore_alloc_read_pointer(scanstate->relation, 0);
+		tuplestore_alloc_read_pointer(scanstate->relation, EXEC_FLAG_REWIND);
 
 	/*
 	 * The new read pointer copies its position from read pointer 0, which