libvpx

Restore first ref frame pointer to the correct value

The joint_motion_search function alternates prediction
between two reference frames. In order to reuse existing
code, a pointer to the appropriate reference frame is
written into xd->plane[0].pre[0], that the motion
estimation code assumes points to the reference frame.

If this first reference frame was scaled then the
pointer was incorrectly being reset to point to the
unscaled reference frame rather than the scaled
version.

Change-Id: I76f73a8d8f4f15c1f3a5e7e08a35140cdb7886ab