vis(3): avoid out-of-bounds stack buffer reads

I found this while running kdump(1) on a CheriBSD system due to a
capability length violation when printing the /etc/libmap.conf read()
system call: it crashed immediately after printing the first line.

Found by: CHERI
Reviewed By: jhb
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D30771

(cherry picked from commit 1a2f06d0f2905c9a18340b377cbbe772f2ca6844)