bug#21881: 24.4; eww-submit is nil

[Eli Zaretskii]

> From: Glenn Morris <rgm@gnu.org>
> Date: Wed, 11 Nov 2015 14:01:43 -0500
> Cc: 21881@debbugs.gnu.org
> 
> Pushpal Sidhu wrote:
> 
> > Running "emacs -q" and using eww, I find that some buttons on some
> > websites cause the following to occur: http://pastebin.com/MtZhnr5e
> >
> > To reproduce easily:
> > 1. point eww to: https://dev.openwrt.org/log/trunk/
> > 2. Select "Show full log messages"
> > 3. Click on the "Update" button.
> >
> > The above steps will cause a wrong-type-argument integer-or-marker-p nil
> > to be thrown. I haven't been able to try 24.5, but I don't think there
> > were any eww changes between 24.4 and 24.5 anyways (reading NEWS.24.5).
> 
> We don't list bug fixes in the NEWS, but by experiment the issue is
> still present in 24.5.

I think I fixed it now.  It was due to attributes that have no value,
so the cdr was nil.

> In current master, I can't even figure out how to toggle a checkbox.
> Neither RET nor mouse-2 works.
> It just beeps "buffer is read-only" at me.

This seems to need binding inhibit-read-only inside eww-update-field.
However, doing that causes incorrect display afterwards, looks like
there's some funky error in computing the string to be inserted
there.  I'm cc'ing Kenjiro NAKAYAMA who made that change.