BUG #14220: pg_get_expr() with an incorrect relation id crashes the server

The following bug has been logged on the website:

Bug reference:      14220
Logged by:          Chris Hanks
Email address:      christopher.m.hanks@gmail.com
PostgreSQL version: 9.5.3
Operating system:   Linux Mint 17.2 64-bit
Description:        

SELECT version():
"PostgreSQL 9.5.3 on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu
4.8.2-19ubuntu1) 4.8.2, 64-bit"

Reproduction:

CREATE TABLE "test_table" ("id" serial PRIMARY KEY, "column_1" int4 NOT
NULL, "column_2" int4);

CREATE INDEX "idx" ON "test_table" ("column_1") WHERE ("column_2" = 4);

SELECT pg_get_expr(i.indpred, i.indexrelid) FROM pg_index i;

I realize that the query is incorrect, in order to retrieve the filter
expression as SQL I need to call pg_get_expr(i.indpred, i.indrelid). But it
seems like it should raise an error, rather than causing the server to
crash.