Fix core dump in left-right screen scrolling for long lines.

The nvi maintainer expects this patch to be included in the upcoming
nvi-1.81.6 unless a better fix presents itself.

The MFC reminder below should be taken as a cue to MFC rev 1.1.1.2
of vs_relative.c as well.

PR: bin/26869
Reported by: Alex Semenyaka <alexs@snark.ratmir.ru>
Obtained from: skimo@kotnet.org (via "Alexey V. Neyman" <alex.neyman@auriga.ru>)
MFC after: 1 month