[libwnck/wip/muktupavels/lp1990263] handle: avoid segfault in invalidate_icons

[Alberts Muktupāvels <muktupavels src gnome org>]

commit e43272333bb6fa16811f4591cef9f9f2f158b8a0
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Tue Sep 20 15:40:22 2022 +0300

    handle: avoid segfault in invalidate_icons
    
    Screens variable won't be initialized if default icon sizes are
    set/changed before using wnck_screen_get_default.
    
    https://bugs.launchpad.net/ubuntu/+source/libwnck3/+bug/1990263

 libwnck/wnck-handle.c | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/libwnck/wnck-handle.c b/libwnck/wnck-handle.c
index e12ff882..8d5da45c 100644
--- a/libwnck/wnck-handle.c
+++ b/libwnck/wnck-handle.c
@@ -70,6 +70,9 @@ invalidate_icons (WnckHandle *self)
   Display *xdisplay;
   int i;
 
+  if (self->screens == NULL)
+    return;
+
   xdisplay = _wnck_get_default_display ();
 
   for (i = 0; i < ScreenCount (xdisplay); ++i)