BUG #14758: Segfault with logical replication on a function index

The following bug has been logged on the website:

Bug reference: 14758
Logged by: Scott Milliken
Email address: scott(at)deltaex(dot)com
PostgreSQL version: 10beta2
Operating system: Linux 4.10.0-27-generic #30~16.04.2-Ubuntu S
Description:

I'm testing logical replication on 10beta2, and found a segfault that I can
reliably reproduce with an index on a not-actually immutable function.

Here's the function in question:

```
CREATE OR REPLACE FUNCTION public.immutable_random(integer)
RETURNS double precision
LANGUAGE sql
IMMUTABLE
AS $function$SELECT random();
$function$;
```

It's not actually immutable since it's calling random (a hack to get an
efficient random sort on a table).

(Aside: I'd understand if it errored on creation of the index, but would
really prefer to keep using this instead of tablesample because it's fast,
deterministic, and doesn't have sampling biases like the SYSTEM sampling.)

Here's full reproduction instructions:

Primary:
```
mkdir -p /tmp/test-seg0
PGPORT=5301 initdb -D /tmp/test-seg0
echo "wal_level = logical" >> /tmp/test-seg0/postgresql.conf
PGPORT=5301 pg_ctl -D /tmp/test-seg0 start
for (( ; ; )); do if pg_isready -d postgres -p 5301; then break; fi; sleep
1; done
psql -p 5301 postgres -c "CREATE USER test WITH PASSWORD 'test' SUPERUSER
CREATEDB CREATEROLE LOGIN REPLICATION BYPASSRLS;"
createdb -p 5301 -E utf8 test

psql -p 5301 -U test test -c "CREATE TABLE testtbl (id int, name text);"
psql -p 5301 -U test test -c "ALTER TABLE testtbl ADD CONSTRAINT
testtbl_pkey PRIMARY KEY (id);"
psql -p 5301 -U test test -c "CREATE PUBLICATION testpub FOR TABLE
testtbl;"
psql -p 5301 -U test test -c "INSERT INTO testtbl (id, name) VALUES (1,
'a');"
```

Secondary:
```
mkdir -p /tmp/test-seg1
PGPORT=5302 initdb -D /tmp/test-seg1
PGPORT=5302 pg_ctl -D /tmp/test-seg1 start
for (( ; ; )); do if pg_isready -d postgres -p 5302; then break; fi; sleep
1; done
psql -p 5302 postgres -c "CREATE USER test WITH PASSWORD 'test' SUPERUSER
CREATEDB CREATEROLE LOGIN REPLICATION BYPASSRLS;"
createdb -p 5302 -E utf8 test

psql -p 5302 -U test test -c "CREATE TABLE testtbl (id int, name text);"
psql -p 5302 -U test test -c "ALTER TABLE testtbl ADD CONSTRAINT
testtbl_pkey PRIMARY KEY (id);"
psql -p 5302 -U test test -c 'CREATE FUNCTION
public.immutable_random(integer) RETURNS double precision LANGUAGE sql
IMMUTABLE AS $function$ SELECT random(); $function$'
psql -p 5302 -U test test -c "CREATE INDEX ix_testtbl_random ON testtbl
USING btree (immutable_random(id));"
psql -p 5302 -U test test -c "CREATE SUBSCRIPTION test0_testpub CONNECTION
'port=5301 user=test dbname=test' PUBLICATION testpub;"
```

The secondary crashes with a segfault:

```
2017-07-23 23:55:37.961 PDT [4823] LOG: logical replication table
synchronization worker for subscription "test0_testpub", table "testtbl"
has started
2017-07-23 23:55:38.244 PDT [4758] LOG: worker process: logical replication
worker for subscription 16396 sync 16386 (PID 4823) was terminated by signal
11: Segmentation fault
2017-07-23 23:55:38.244 PDT [4758] LOG: terminating any other active server
processes
2017-07-23 23:55:38.245 PDT [4763] WARNING: terminating connection because
of crash of another server process
2017-07-23 23:55:38.245 PDT [4763] DETAIL: The postmaster has commanded
this server process to roll back the current transaction and exit, because
another server process exited
abnormally and possibly corrupted shared memory.
2017-07-23 23:55:38.245 PDT [4763] HINT: In a moment you should be able to
reconnect to the database and repeat your command.
2017-07-23 23:55:38.247 PDT [4758] LOG: all server processes terminated;
reinitializing
2017-07-23 23:55:38.256 PDT [4826] LOG: database system was interrupted;
last known up at 2017-07-23 23:55:36 PDT
2017-07-23 23:55:38.809 PDT [4826] LOG: database system was not properly
shut down; automatic recovery in progress
2017-07-23 23:55:38.812 PDT [4826] LOG: redo starts at 0/173AEA0
2017-07-23 23:55:38.815 PDT [4826] LOG: invalid record length at 0/17B50B0:
wanted 24, got 0
2017-07-23 23:55:38.815 PDT [4826] LOG: redo done at 0/17B5070
2017-07-23 23:55:38.815 PDT [4826] LOG: last completed transaction was at
log time 2017-07-23 23:55:37.962957-07
```