x11-wm/fvwm3: Avoid crash in FvwmIconMan

Add a patch working around the crash by initializing some struct member
to NULL that's otherwise used uninitialized and backporting some code
from upstream's main branch to correctly handle that case.

PR: 265291

(cherry picked from commit 1b4b3e9ce1e7255519f84324f3a7a4c1d7cd3f33)