pango_fc_family_list_faces doesn't set the faces and n_faces arguments when the fontmap is NULL
pango_fc_family_list_faces
returns immediately if !fcfamily->fontmap
, but then the pointers in faces
abd n_faces
are left pointing to whatever they used to.
It used to do
*faces = NULL;
*n_faces = 0;
But that was removed in c40f0f53
Maybe better to do this settings in pango_font_family_list_faces
.