Should not allow free of already freed scratch ndarray

[jalava]

Had very strange results after calling free twice on same ndarray. After that pool.zeroes started returning buffers that were currently in use.

To prevent this, I think you should keep flag with ndarray of it's malloc state and throw error if you are trying to free already released buffer.

[mikolalysenko]

Maybe we could make a debug version of ndarray-scratch which tests to see if an array was double free'd.