Minor optimizations for the unsafe version

benchmark                           old ns/op     new ns/op     delta
BenchmarkXxhash32                   2131          1907          -10.51%
BenchmarkXxhash64                   1192          1097          -7.97%
BenchmarkXxhash64VeryShort          17.0          19.3          +13.53%
BenchmarkXxhash64MultiWrites        1455          1352          -7.08%