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