The new scheme introduced in I68d35a2f did not clamp chroma MVs in the SPLITMV
case, and clamped them incorrectly (to the luma plane bounds) in every other
case.
Because chroma MVs are computed from the luma MVs before clamping occurs, they
could still point outside of the frame buffer and cause crashes.
This clamping happens outside of the MV prediction loop, and so should not
affect bitstream decoding.
05c6eca4db