Don't check corner level 3 MVs.
Don't check corner level 3 MVs.

This bug never triggered since the encoder is careful to set these
corner level 3 MV flags to invalid. We were erroneously looking at them
even though they were always invalid and would have coded a tristate and
perhaps peeked outside the grid to do so.

Spotted by Tim during review of https://review.xiph.org/451/

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