x11-wm/xfce4-wm: Import upsstream patch fixing crashs

xfce4-wm could crash if a broken client tries to usse a nonexistent
icon. Importing upstream patch that avoids the crash.

PR: 262908
Obtained from: https://gitlab.xfce.org/xfce/xfwm4/-/commit/a7971823de40a17dcba940fcef56d781626826d3
MFH: 2022Q1

(cherry picked from commit 45e9c8923b1f38f856b2df9b7385cabbff95d82c)