Alter webvtt_release_node to set the node's pointer to null.

- I've altered webvtt_release_node to set the caller's reference to null. This should avoid
problems where we think we haven't released a node, but we have.
- I've also changed webvtt_create_node_from_token to return unsuccessful if it is passed a
node that is not null. This should ensure that we are not overwritting the callers reference
before it releases it.