[Pie Charts] Destroying chart while the chart is animating causes null error.

* **c3 v0.4.17**:
* **d3 3.5.17**:
* **Chrome**:
* **Mac OSX**:

When .destroy() is called on a pie chart while it is animating (you have to be lucky and time this one accurately), you can get a null pointer exception. Because .destroy() sets every property of $$ to null, near-future calls to stuff like $$.data.targets or $$.config.data_types will throw a null error since $$.data and $$.config are set to null.