If the intrabc motion vectors are invalid or out of range then
this patch defines deterministic decoder behaviour (disabling intrabc).
This will then allow conformance tests to be written to check this
deterministic behaviour so that we do not end up with undefined
behaviour or compatibility problems related to illegal intrabc
motion vectors.

Change-Id: I4de3e006eaf01ab07a7fbbbfd7cfe28b0b0d140c