Don't run text off a page when longtable is used. (#434)

This bug was reported (with a fix) way back in 2003 (7 years before
RISC-V was started at Berkeley) at
https://www.latex-project.org/cgi-bin/ltxbugs2html.new?pr=tools/3512

For us, it should fix #432. I'll apply this to the release branch once
it merges to master.