Fix crashing bug

Yes, we have to manually vivify, and yes, we have to bind instead of
assign to do it. Don't ask why assignment fails in this one spot
(whether using autovivification or doing that manually), in specific
conditions.

specs commit 562ef2c and roast commit 4a5a180 were used when the weird
failure was occurring; use those if you can't reproduce these problems
in the future (which occurs during the first can't-find-section backlink
for S02).