fix for closing db in sql worker

[WilliamStephens]

Null the db after closing it. This was causing issues when we closed a db and then reopened it because db was never set to null.

[dinedal]

Hi @WilliamStephens first thank you very much for contributing!

I know it can be a bit confusing, but the actual logic for the JS files is written in CoffeeScript, and the worker's logic lives here: https://github.com/kripken/sql.js/blob/master/coffee/worker.coffee

After you make the change to that file, make worker will generated the correct worker.sql.js

[WilliamStephens]

@dinedal Yea no problem!

After an hour of setup I think I finally got it. When I ran the make file it generated a new sql.js and worker.sql.js and caused a some big diffs that I didn't include. You may want to handle that on your end as I'm not really sure if they are needed or not. I ran the test.sh script and had a bunch of failures but honestly am not sure if I caused those or not.

First time writing Coffeescript too, so not sure if there is a better way to do this.

[dinedal]

Yeah we need the regenerated source as well. Let me see if I can help out later

[WilliamStephens]

@dinedal Added the regenerated files