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