OID min/max functions

I noticed that there was an item in the TODO about min/max for OIDs.
Thought that maybe the new type coersion stuff would take care of it by
converting OID to int4. It probably did, but something else is unhappy:

postgres=> select min(oid) from t;
pqReadData() -- backend closed the channel unexpectedly.

postgres=> select min(x) from t;
min
---
a
(1 row)

postgres=> select int4(oid) from t;
?column?
--------
18570
18571
18572
144716
(4 rows)

postgres=> select min(int4(oid)) from t;
pqReadData() -- backend closed the channel unexpectedly.

Oh well. I'll put it on my list of things to look at...

- Tom