[m-rev.] for post-commit review: fix autopar feedback aborts

[Julien Fischer]

Hi Zoltan,

On Thu, 15 Aug 2019, Zoltan Somogyi wrote:

> What I don't like about the current rules is that while fetch_items
> does bounds checks, in this presumably reasonably common scenario
> it requires its caller to do a bounds check as well. However, I don't
> have *that* strong an opinion about it, so I will let it rest there.

Within the confines of the Mercury system there are four uses of
fetch_items.  The one in library/io.m cannot encounter this issue.
Another two are the ones in are the ones in the autopar code and there's
a further instance in extras/lex.  The latter requires the same
behavious as the autopar calls.

I'll swap the order of the tests in fetch_items around.

Julien.