array_in() and array_recv() need to be more paranoid about validating

array_in() and array_recv() need to be more paranoid about validating
their OID parameter.  It was possible to crash the backend with
select array_in('{123}',0,0); because that would bypass the needed step
of initializing the workspace.  These seem to be the only two places
with a problem, though (record_in and record_recv don't have the issue,
and the other array functions aren't depending on user-supplied input).
Back-patch as far as 7.4; 7.3 does not have the bug.