MF stable/12 r352534:

Fix an off-by-one error from r351961

That revision addressed a Coverity CID that could lead to a buffer overflow,
but it had an off-by-one error in the buffer size check.

Approved by: re (kib)
Reported by: Coverity
Coverity CID: 1405530
MFC-With: 351961
Sponsored by: The FreeBSD Foundation