easy crash on HEAD

I am pretty sure this is an easily-reproducible crash on git head
(well, as of a2a480af889b5), helpfully confirmed on IRC by wulczer and
deko. I reproduced the crash myself on OS X and 64-bit Debian.

---
create table foo (a int);
CREATE RULE notify_foo_updates AS ON UPDATE TO foo DO NOTIFY foo;
\d foo
---

Backtrace looks like this:
(gdb) bt
#0 0x00007fff837ce954 in recvfrom ()
#1 0x0000000100188fe3 in secure_read (port=0x100802280,
ptr=0x10055d340, len=8192) at be-secure.c:304
#2 0x0000000100191bdc in pq_recvbuf () at pqcomm.c:854
#3 0x0000000100192053 in pq_getbyte () at pqcomm.c:895
#4 0x0000000100257fec in SocketBackend [inlined] () at
/media/src/OSS/postgresql/src/backend/tcop/postgres.c:344
#5 0x0000000100257fec in ReadCommand (inBuf=0x7fff5fbfe0d0) at postgres.c:492
#6 0x0000000100258746 in PostgresMain (argc=1, argv=0x10101a010,
dbname=0x101019e78 "adsync", username=<value temporarily unavailable,
due to optimizations>) at postgres.c:3946
#7 0x0000000100204a95 in BackendRun [inlined] () at
/media/src/OSS/postgresql/src/backend/postmaster/postmaster.c:3985
#8 0x0000000100204a95 in ServerLoop () at postmaster.c:3674
#9 0x00000001002083d4 in PostmasterMain (argc=3, argv=0x100800700) at
postmaster.c:1244
#10 0x0000000100194089 in main (argc=3, argv=0x100800700) at main.c:196

Josh