bug#36403: 27.0.50; Trivial image.c bugs

[Eli Zaretskii]

> From: Pip Cet <pipcet@gmail.com>
> Date: Thu, 27 Jun 2019 16:28:05 +0000
> 
> I think with the exception of the contrived last example, these are
> all easy to fix, but a bit harder to fix well. I've tried to do the
> former, for now, but I'd welcome any help for me to do the latter.

Thanks, but please also add tests which verify that these problems no
longer happen after the fix.