LibWeb: Update m_previous_sibling in TreeNode::insert_before.

Author: https://github.com/asynts
Commit: https://github.com/SerenityOS/serenity/commit/4eb23abf065
Pull-request: https://github.com/SerenityOS/serenity/pull/4182

Libraries/LibWeb/TreeNode.h

@@ -380,6 +380,8 @@ inline void TreeNode<T>::insert_before(NonnullRefPtr<T> node, RefPtr<T> child, b
     node->m_previous_sibling = child->m_previous_sibling;
     node->m_next_sibling = child;
 
+    child->m_previous_sibling = node;
+
     if (m_first_child == child)
         m_first_child = node;