bug#51105: 29.0.50; Buffer overflow bug in ns_compute_glyph_string_overhangs

[Eli Zaretskii]

> From: Daniel Martín <mardani29@yahoo.es>
> Cc: 51105@debbugs.gnu.org
> Date: Sat, 09 Oct 2021 12:06:36 +0200
> 
> Now I think that the right thing to do may be to modify nsterm.m, switch
> on the glyph type and, if the glyph type is COMPOSITE_GLYPH, call
> composition_gstring_width to get the glyph metrics.  Function
> composition_gstring_width uses the values from fields s->cmp_from and
> s->cmp_to, and would avoid the buffer overflow:
> 
> (lldb) fr v s->cmp_from
> (int) s->cmp_from = 6
> (lldb) fr v s->cmp_to
> (int) s->cmp_to = 7
> 
> WDYT? I can prepare a patch of this type if you agree.

SGTM, but I'd like to hear Alan's opinion as well, as I don't feel I
know enough about the NS display backend.