Fix SSI-related assertion failure.

Bug #5899, reported by Marko Tiikkaja.

Heikki Linnakangas, reviewed by Kevin Grittner and Dan Ports.

Author: robertmhaas

src/backend/access/heap/heapam.c

@@ -2859,7 +2859,7 @@ heap_update(Relation relation, ItemPointer otid, HeapTuple newtup,
 	 * Any existing SIREAD locks on the old tuple must be linked to the new
 	 * tuple for conflict detection purposes.
 	 */
-	PredicateLockTupleRowVersionLink(relation, &oldtup, newtup);
+	PredicateLockTupleRowVersionLink(relation, &oldtup, heaptup);
 
 	if (newbuf != buffer)
 		LockBuffer(newbuf, BUFFER_LOCK_UNLOCK);