xd->plane[0].n4_h and xd->plane[0].n4_w are not set at that point
when using supertx.

While this fixes the immediate crash described in the referenced
bug report, there are still issues in the ref-mv experiment that
causes these tests to fail, so they are kept disabled.

BUG=https://bugs.chromium.org/p/webm/issues/detail?id=1230

Change-Id: Ibf8ef02847a903f8d10e6be28e16694db10c75af