fix the mv_ref_idx issue

The following issue was reported :
https://code.google.com/p/webm/issues/detail?id=601&q=jimbankoski&sort=-id&colspec=ID%20Pri%20mstone%20ReleaseBlock%20Type%20Component%20Status%20Owner%20Summary

This code makes the choice and code cleaner and removes any question
about whether the border needs to be checked.

Change-Id: Ia7aecfb3168e340618805bd318499176c2989597