Fix crash after flush cache (#12107)

Co-authored-by: fzyzcjy <5236035+fzyzcjy@users.noreply.github.com>

python/sglang/srt/mem_cache/radix_cache.py

@@ -533,6 +533,10 @@ class RadixCache(BasePrefixCache):
                 self.protected_size_ -= len(node.key)
                 delta += len(node.key)
             node.lock_ref -= 1
+            if node.parent is None:
+                assert (
+                    node is self.root_node
+                ), f"This request holds the node from another tree"
             node = node.parent
         return delta