Fix to double deletion of StreamingZipInflater object.

Race condition can cause double deletion during stability runs
of StreamingZipInflater object. This is possible due to delete
of mZipInflater occuring at two seperate locations in owning
_CompressedAsset object. Fixed by securing object is set to
null on member reference before calling delete. Also secured
that all deleted objects in StreamingZipInflater are set to
null after delete.

Change-Id: I87ef0fa78835ea05c524b3d818c8e6aa2aee1fe4