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

[markj]

Summary

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 <[e-mail]>

[kib]

sys/libkern/strndup.c

43–44

It would be cleaner to stop adding 1 to len. There is only one use of 'strnlen + 1' value, in malloc. There, you can add 1 explicitly.

Then the code becomes similar (if not identical) to the libc version.

[markj]

Take kib's suggestion