[BUG] half_siphash_32 does not work correctly with 8-byte key

[jiachin1995]

Issue: When using 8-byte key, half_siphash_32() changes output depending on when and where its called.

Suspected Cause: C library function is reading memory from the 9th byte onwards.

Steps to reproduce bug:

# Run the following code
from siphash import half_siphash_32

data = b'PQWXWDRRASD2164DW5G'
key = b'\x124Vx'     # 8-byte key
half_siphash_32(key, data)
key = b'\x124Vx'
half_siphash_32(key, data)
key = b'\x124Vx'
half_siphash_32(key, data)
key = b'\x124Vx'
half_siphash_32(key, data)

My output:

>>> from siphash import half_siphash_32
>>>
>>> data = b'PQWXWDRRASD2164DW5G'
>>> key = b'\x124Vx'
>>> half_siphash_32(key, data)
b'\xb73(\x1c'
>>> key = b'\x124Vx'
>>> half_siphash_32(key, data)
b'\x15\xcf\x1e\xa3'
>>> key = b'\x124Vx'
>>> half_siphash_32(key, data)
b'\xb73(\x1c'
>>> key = b'\x124Vx'
>>> half_siphash_32(key, data)
b'\x15\xcf\x1e\xa3'

As shown above, half_siphash_32 returns b'\x15\xcf\x1e\xa3' or b'\xb73(\x1c' randomly