bug#6214: 23.1; json-read-string crashes emacs with long string

[Chong Yidong]

Carl Worth <cworth at cworth.org> writes:

> A user of the emacs-based mail client, Notmuch [*], found that
> attempting to display a particular message would consistently
> causes a segmentation fault in emacs.
>
> I haven't attempted to debug this within gdb yet, (I'll have to get my
> hands on a build of emacs with debugging symbols first). But I wanted to
> share things right away, so that perhaps someone else could do further
> debugging and follow up.

Looks like a stack overflow in the `string' function.  I've checked in a
fix, thanks for the bug report.