bug#58028: 29.0.50; Ensures no leakage of glyph_matrix

[Eli Zaretskii]

> From: Manuel Giraud <manuel@ledu-giraud.fr>
> Date: Fri, 23 Sep 2022 17:22:07 +0200
> 
> Maybe this problem never triggers.  It is a « better be safe » kind of
> patch.

Please add an assertion there, to catch the case that previously was
handled by testing only w->desired_matrix.  It might be that what you
think is a memory-leak bug is in fact a feature.

Thanks.