fix the error/memleak in Firefox 3.0.x and 25.0.1 concerning getBBox

Firefox 25.0.1 try to stop the script

```
A script on this page may be busy, or it may have stopped responding. 
You can stop the script now, or you can continue to see if the script will complete [...]
```

Debugging I was able to find out, that the problem was caused by the method _getBBox.
I provided to fix the error according to the code used to solve the same problem in SVG.js [here](https://github.com/wout/svg.js/issues/21).