x11-fonts/fontconfig: webfonts on many sites crash www/firefox

A "bad" commit [1] included in the recent fontconfig upgrade to 2.13.92, lead to many
crashed tabs inside firefox. A fix has been provided upstream in [2], and fixed in fix [3].

Related Mozilla bugreport [4].

[1] https://cgit.freedesktop.org/fontconfig/commit/?id=61573ad5f7c4dd0860d613d99d0086433240eb75
[2] https://gitlab.freedesktop.org/fontconfig/fontconfig/-/issues/237
[3] https://gitlab.freedesktop.org/fontconfig/fontconfig/-/commit/6edaaa4d1823518a97fb4cc3004d110b1046f742
[4] https://bugzilla.mozilla.org/show_bug.cgi?id=1629231

PR: 245915
Submitted by: jbeich
Reported by: Graham Perrin, jbeich, ehaupt, tcberner