fix string used after freed problem

new string created in topic tree (#137)

*Issue #, if available:*
https://github.com/awslabs/aws-c-mqtt/issues/135

*Description of changes:*
- subscribe_task_topic takes ownership of the topic string. And the nodes in the topic tree will create a new string now, which means the caller of the aws_mqtt_topic_tree_insert() will need to clean up the topic_filter.

Co-authored-by: Dengke Tang <dengket@amazon.com>