bug#42904: [PATCH] Non-Unicode frame title crashes Emacs on macOS

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Tue, 18 Aug 2020 13:48:10 +0200
> Cc: Eli Zaretskii <eliz@gnu.org>, 42904@debbugs.gnu.org
> 
> Anyway, here is an alternative patch using your method. Tell us what you 
> think.

Thanks, but I don't think we should modify the :default-char attribute
of the UTF-* encodings as part of this change.  It's a separate issue,
and is a backward-incompatible change of sorts.  For instance, we
should consider what this will do to display on TTY frames that don't
support Unicode.  So I think we should discuss this issue separately
before we make such a change.

Why is it a problem to display a space instead of invalid bytes in
this case?