Add safety check for QueryTree

xcb_qeuery_tree should fail and we should not just assume that the
call succeeds.

Crash reported by Harald Sitter.

layers.cpp

@@ -668,6 +668,7 @@ ToplevelList Workspace::xStackingOrder() const
     foreach (Toplevel * c, stacking_order)
     x_stacking.append(c);
 
+    if (!tree.isNull()) {
         xcb_window_t *windows = tree.children();
         const auto count = tree->children_len;
         int foundUnmanagedCount = unmanaged.count();
@@ -686,6 +687,7 @@ ToplevelList Workspace::xStackingOrder() const
                 break;
             }
         }
+    }
     if (m_compositor) {
         const_cast< Workspace* >(this)->m_compositor->checkUnredirect();
     }

xcbutils.h

@@ -574,7 +574,7 @@ public:
     explicit Tree(WindowId window) : Wrapper<TreeData, xcb_window_t>(window) {}
 
     inline WindowId *children() {
-        if (data()->children_len == 0) {
+        if (isNull() || data()->children_len == 0) {
             return nullptr;
         }
         return xcb_query_tree_children(data());