libkern: Avoid a one-byte OOB access in strndup()

If the length of the string is maxlen, we would end up copying maxlen+1
bytes, which violates the contract of the function. The result is the
same since that extra byte is overwritten.

Reported by: Kevin Day <kevin@your.org>
Reviewed by: imp, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D54093