fix for memory leak in dataconnection.js

Issue: https://github.com/peers/peerjs/issues/200

After completing a large file transfer, it seems like the `_chunkedData` object is not being cleared frequently enough, and so it accumulates data and eats up memory. I think the issue is due to the fact that `delete this._chunkedData[id];` is executed after the recursive call to `_handleDataMessage()`.

To fix, execute `delete this._chunkedData[id];` before the recursive call to `_handleDataMessage()` to free up memory and prevent `_chunkedData` from accumulating a bunch of data.