Avoid reading one byte before the path buffer.

This happens when there's only one component (e.g. "/foo"). This
(mostly-harmless) bug has been present since June 1990 when it was
commited to mountd.c SCCS version 5.9.

Note: the bug is on the second changed line, the first line is changed
for visual consistency.

Reviewed by: cem, emaste, mckusick, rmacklem
Found with: CHERI
Obtained from: CheriBSD
MFC after: 1 week
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D25759