vt: bound buffer access in redraw optimization

PR: 248628
Reported by: oleg
Reviewed by: cem, oleg (both earlier)
Fixes: ee97b2336aa4 ("Speed up vt(4) by keeping...")
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32059

(cherry picked from commit dbc7ca59451561a179f9852642e13ef024169d84)
(cherry picked from commit e4fcff8ee124f8faed2f1fcc1e15b7ae6906d4d7)

Approved by: so
Security: FreeBSD-SA-22:01.vt