If a parent mb is available but is intra coded, then parent_ref_mv is
invalid. Check that the parent is inter coded before trying to access
the parent_ref_mv. Previously the parent_ref_mv was being read from
an uninitialized stack allocation, causing potential OOB reads and
other undefined behavior.

Change-Id: I0c93cd412a19c3a184bcf6decaa145b3a036a6c0