Fix above context pointers

Fix above context pointers

In the prior code, the above context pointers used for entropy
decoding were initialized on the first frame, and not updated when
the frame size changed. The per-frame code which initializes the
contexts assumes that the contexts are contiguous, leading to an
incomplete initialization when the frame is smaller. This commit
updates the pointers so that the context is contigous whenever
the frame size changes.

Change-Id: I08b53e3a30c8289491212311682ff1b8028cff6c