Context-pred fix to not use top/left on edges

This fix resolves some of the mismatch issues being seen
recently. While this is the right thing to do when tiling
is used for this experiment, it is not the underlying cause
of the the mismatches.
Something else is causing writing outside of the allowable
frame area in the encoder leading to this mismatch.

Change-Id: If52c6f67555aa18ab8762865384e323b47237277