Fix warning in ext_getPropertyAccessorsForClass

I'm getting the following warning in Xcode 6 beta 6:

> Pods/libextobjc/extobjc/EXTRuntimeExtensions.m:710:18: Initialization of pointer of type 'ext_propertyAttributes *' to null from a constant boolean expression

![screen shot 2014-08-20 at 10 39 42 am](https://cloud.githubusercontent.com/assets/442307/3985395/4a59b642-2891-11e4-8e2c-2e43a8d4a843.png)

My understanding is limited, but it seems as though `attributes` is no longer used after the `free(attributes)` line. My changes fix the warning, but would this change have any unintended consequences? I ran the tests and they pass (though they fail to compile without the fix due to the warning).