When coding the frame that corresponds to the midpoint frame
defining an ARF, do not update the last reference frame buffer.
Previously this buffer was updated meaning that when coding the next
ARF all the reference buffers were the same (or nearly so).
Turning the update off means that the frame before is still available
as an alternative predictor and for use in compound prediction.

Also fixed inconsistency in test for mismatch (patch from JK).

Net average gains (derf 0.049, yt 0.163, yt-hd 0.207, std-hd 0.286)

Change-Id: Ifee21da21ccbb1648ac2eafe890d3ce60562c7bc