Re: memory leak in pgoutput

> Here, after freeing the tupledesc, the ExecDropSingleTupleTableSlot will still
> access the freed tupledesc->tdrefcount which is an illegal memory access.

Yes, I overlooked that.

> I think we can do something like below instead:
>
> + TupleDesc desc = entry->old_slot->tts_tupleDescriptor;
> +
> + Assert(desc->tdrefcount == -1);
> +
> ExecDropSingleTupleTableSlot(entry->old_slot);
> + FreeTupleDesc(desc);

It seems a bit odd because "entry->old_slot->tts_tupleDescriptor" is accessed
after "entry->old_slot" has been freed. I think we can avoid this by assigning
"desc" to NULL before ExecDropSingleTupleTableSlot().

```
+ TupleDesc desc = entry->old_slot->tts_tupleDescriptor;
+
+ Assert(desc->tdrefcount == -1);
+
+ FreeTupleDesc(desc);
+ desc = NULL;
ExecDropSingleTupleTableSlot(entry->old_slot);
```

By the way, this issue is introduced in 52e4f0cd472d39d. Therefore, we may need
to backport the patch to v15.

Best Regards,
Boyu Yang