x11/fvwm3: Fix FvwmIconMan module segfault
Initialize the offending field in the fscreen_scr_arg struct to NULL,
fixing the segfault.
PR: 265291
Submitted by: Felix Palmen <felix@palmen-it.de> (Maintainer)
Reported by: anubhav.x61+freebsd@gmail.com
Approved by: Felix Palmen <felix@palmen-it.de> (Maintainer)