LibWeb: Detach the paint tree in Document::tear_down_the_layout_tree()

Otherwise, someone may try to paint the paint tree after the layout tree
has been torn down. This could in theory be possible in the future, but
until we've completely severed the paint tree's dependency on the layout
tree, this is not safe as the layout tree is no longer in a valid state.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -810,6 +810,8 @@ void Document::tear_down_layout_tree()
     }
 
     m_layout_root = nullptr;
+
+    m_paintable = nullptr;
 }
 
 Color Document::background_color() const