[NORMATIVE] Set cm->prev_frame = NULL at keyframes
[NORMATIVE] Set cm->prev_frame = NULL at keyframes

For non-key frames, the function get_prev_frame() takes care of
deciding what cm->prev_frame should be. But we are currently not
explicitly setting cm->prev_frame at keyframes.

This leads to the correct behaviour for the first keyframe in a
stream, since cm->prev_frame is initialized to NULL before decoding
anything, but later keyframes get wrong results since some code now
does different things depending on whether cm->prev_frame is NULL
or not.

Fix this by explicitly setting cm->prev_frame = NULL at keyframes.

BUG=aomedia:1601

Change-Id: Ibc9847945484142fc8d9a28d27aa1131f3c604f5

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom