bug#7908: png-1.5 fix for emacs-23.2 and HEAD

[Chong Yidong]

Eli Zaretskii <eliz@gnu.org> writes:

>> Aha, I think it makes sense.  Since DEF_IMGLIB_FN defines a return type
>> of int, whereas fn_png_set_longjmp_fn actually returns a (jmp_buf *), we
>> do indeed need to perform the extra cast.  Well spotted.  Does Emacs
>> compile properly with this change?
>
> Yes, it does.

Excellent.  I've committed the corrected patch.  Thanks very much for
helping fix this.