siphash: allow zero values for final & len in SipBuf()

Currently the assert checks for XOR of final and len.
This assert fails when running the unit tests in siphash_test.c.
We need to allow the case where both values are zero.

Signed-off-by: Mark O'Donovan <shiftee@posteo.net>
Reviewed by: imp, cperciva
Pull Request: https://github.com/freebsd/freebsd-src/pull/1324