Currently the RD loop traverses 4X8 blocks in inverted N order while
the bitstream stores blocks smaller than 8x8 in Z order. This causes a
discrepancy where the RD loop reads uninitialized data while
performing intra prediction.  As a temporary fix simply disable the
use of the extended right edge for 4X8 blocks, until the bitstream can
be changed to match the logical structure of the blocks.

Change-Id: I44a9e4fc1a15cd551a7b38c3c1227bc5dac77e9a