Avoid reading one byte before the path buffer.

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