cache lookup failed for statistics object 123

Per sqlsmith.

postgres=# SELECT pg_get_statisticsobjdef_expressions(123);
ERROR:  cache lookup failed for statistics object 123
postgres=# \errverbose 
ERROR:  XX000: cache lookup failed for statistics object 123
LOCATION:  pg_get_statisticsobjdef_expressions, ruleutils.c:1762

The expectation is that sql callable functions should return null rather than
hitting elog().

In the 003 patch, I wonder if this part should be updated, too:

| ... which can greatly improve query plans that use the expression index.

It can improve queries even that don't use the index, right ?
Say, if a query has f(x) = 11, and the MCV list for the expression shows that
50% of the table has f(x)=11, then the query might decide to *not* use an index
scan.

-- 
Justin