x11-wm/xfce4-panel: Import upstream commit to fix crash

Reported by: hselasky
Obtained from: https://gitlab.xfce.org/xfce/xfce4-panel/-/commit/3bf948cc2618632aaaaabefae1b1be50d5a607c3

(cherry picked from commit 4ce9f853281de004da253d761a4ef81a0b427e7f)