bug#36403: 27.0.50; Trivial image.c bugs

[Eli Zaretskii]

> From: Pip Cet <pipcet@gmail.com>
> Date: Fri, 28 Jun 2019 15:05:30 +0000
> Cc: 36403@debbugs.gnu.org
> 
> > Thanks, but please also add tests which verify that these problems no
> > longer happen after the fix.
> 
> Thanks for reminding me of that! I looked into that, and it seems
> there are no image tests so far.

There are some in test/manual.  they are there because they need to be
run interactively.

> Attached patch has tests and fixes.

Thanks.  Let's wait for a few days to let others comment.