Do not execute callback with unparsed JSON when failed to parse _session_store.json

I updated to Geddy 13.0.7 and got following error when I open a page.

    Could not parse session store, using empty session
    [Thu, 06 Nov 2014 09:57:02 GMT] ERROR 127.0.0.1 - - [Thu Nov 06 2014 18:57:02 GMT+0900 (JST)] "GET / 1.1" 500 1031 "-" "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:33.0) Gecko/20100101 Firefox/33.0"
    TypeError: Cannot set property 'accessTime' of undefined
    at set (/home/phanect/.nvm/v0.10.33/lib/node_modules/geddy/lib/sessions/index.js:135:20)
    at utils.mixin.beginOutput (/home/phanect/.nvm/v0.10.33/lib/node_modules/geddy/lib/controller/base_controller.js:508:20)
    at utils.mixin.output (/home/phanect/.nvm/v0.10.33/lib/node_modules/geddy/lib/controller/base_controller.js:537:10)
    at cb (/home/phanect/.nvm/v0.10.33/lib/node_modules/geddy/lib/controller/base_controller.js:659:16)
    at /home/phanect/.nvm/v0.10.33/lib/node_modules/geddy/lib/template/partial.js:146:13
    at Partial.renderChildren (/home/phanect/.nvm/v0.10.33/lib/node_modules/geddy/lib/template/partial.js:152:7)
    at handleData (/home/phanect/.nvm/v0.10.33/lib/node_modules/geddy/lib/template/partial.js:100:16)
    at /home/phanect/.nvm/v0.10.33/lib/node_modules/geddy/lib/template/partial.js:117:9
    at fs.js:271:14
    at Object.oncomplete (fs.js:107:15)

The cause is old _session_store.json. The content of _session_store.json was invalid JSON.
The content was:

    ""SiZHqfYjR6eSOJbmaHR1kAlPAEHJK8EG8tWOAuwJJf8j7BRwVbNuwVZ22Gg245kWpgfzGBqXAy3w2sjx38WF4ByqAWcg0HM6IjxzZdLIQklJoXDETU0DFh27OiYFxkfg":{"accessTime":1415166423222,"flashMessages":{}}}

So `JSON.parse` in _getDatastore is failed and go to catch block. `callback(null, {});` is executed.
However, there is `callback(null, data);` after it and it is executed.

Then unparsed data is passed to the callback function as an argument, and above error occured.