opr: Use opr_Assert() instead of silently failing

opr: Use opr_Assert() instead of silently failing

These routines should never be passed a NULL.  If this happens it
is a serious issue that needs to be addressed.

Change-Id: I9728dcd67bc9f8e9927bed1674fc0ee83567df1a
Reviewed-on: http://gerrit.openafs.org/11853
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>