[xml-conduit] Fix space leak.

Using xml-conduit to parse some larger XML files I've noticed progressively increasing memory use and slowing. For a 2.5MB file I produced the following heap profile:
![Original heap profile](https://f.cloud.github.com/assets/798147/769266/350e98fa-e8b1-11e2-9c06-4b82eb44a425.png)

I managed to track the space leak down to `toEventC` and `tokenToEvent` and I believe the attached patch is the minimal change necessary to avoid the leak. With the change the heap profile for the same XML file now appears as follows:
![Improved heap profile](https://f.cloud.github.com/assets/798147/769312/2bd6e41c-e8b2-11e2-9887-ffa92791b651.png)