amvalidate(): cache lookup failed for operator class 123

Per sqlsmith.

postgres=# select amvalidate(123);
ERROR:  cache lookup failed for operator class 123
postgres=# \errverbose 
ERROR:  XX000: cache lookup failed for operator class 123
LOCATION:  amvalidate, amapi.c:125

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

-- 
Justin