Possible SSI bug in heap_update

While re-reading heap_update() in connection with that PANIC we're
chasing, my attention was drawn to this comment:

/*
* Note: beyond this point, use oldtup not otid to refer to old tuple.
* otid may very well point at newtup->t_self, which we will overwrite
* with the new tuple's location, so there's great risk of confusion if we
* use otid anymore.
*/

This seemingly sage advice is being ignored in one place:

CheckForSerializableConflictIn(relation, otid, BufferGetBlockNumber(buffer));

I wonder whether that's a mistake. There'd be only a low probability
of our detecting it through testing, I fear.

regards, tom lane