bug#39892: 28.0.50; Crash when running async command

[Robert Pluim]

>>>>> On Thu, 05 Mar 2020 11:46:59 +0200, Eli Zaretskii <eliz@gnu.org> said:

    Eli> How about the patch below?  It works for me, and seems to be TRT in
    Eli> general, since it makes the code more defensive in the face of
    Eli> possible nil values in that vector, which are legitimate according to
    Eli> my reading of the commentary at the beginning of fontset.c.

That patch works for me.

    >> Btw, U+E9F8 is a PUA character; how did it get into a context where we
    >> are trying to display it?

    Eli> I'd still like to know the answer to this.  It seems like something
    Eli> fishy is going on in the original use case that requires us to try to
    Eli> display this PUA codepoint.

Itʼs essentially doing 'cat' on a .gz file. Thereʼs no guarantee
whatsoever that the result is valid. And in fact doing exactly that
crashes the same way (without the patch).

Interestingly, under macOS it displays a proper glyph using
"mac-ct:-*-Hannotate TC-normal-normal-normal-*-14-*-*-*-p-0-iso10646-1
(#x933F)", so somebody has decided that codepoint is good for something.

Robert