internal: fix crash at startup on FreeBSD (#12298)

Hyprland at the latest commit crashes at starting up on FreeBSD with
SIGSEGV. Checking the validity of g_pXWayland->m_wm before calling
updateWorkArea() appears to fix the issue.

src/Compositor.cpp

@@ -3029,6 +3029,8 @@ void CCompositor::arrangeMonitors() {
 
 #ifndef NO_XWAYLAND
     CBox box = g_pCompositor->calculateX11WorkArea();
+    if (!g_pXWayland || !g_pXWayland->m_wm)
+        return;
     g_pXWayland->m_wm->updateWorkArea(box.x, box.y, box.w, box.h);
 #endif
 }