suzerain: eagerly defend against usage error

Spent half an hour looking at the wrong source of a NULL.
Add an always-active ENSURE and sometimes active asserts
to not do that again.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@45120 0bd85f0b-c151-43b9-a1e9-27231104551f