visit_reachable: replace release-mode test with an assert. (GH-16866)

visit_reachable: replace release-mode test with an assert. (GH-16866)

It should be impossible for an untracked object to have the collecting
flag set.  Back when state was stored in gc_refs, it obviously was
impossible (gc_refs couldn't possibly have a positive & negative value
simultaneously).  While the _implementation_ of "state" has gotten much
more complicated, it's still _logically_ just as impossible.