Make the api behavior conform to api spec.
When no more data is available, vpx_codec_decode should be called with NULL as data and 0 as data_sz. vpx_codec_get_frame iterates over a list of the frames available for display. The iterator storage should be initialized to NULL to start the iteration. Iteration is complete when this function returns NULL. Also change the unit test to conform to the api spec. Change-Id: I4b258b309f5df3d37d10c82f01492c0394181c2a