Don't pass passwords with NULL to libscrypt

Cherry-picked from master:
d53f0d84894645be5679088575f7b6c3fb6f7199

Conflicts:
	pylibscrypt/pylibscrypt.py
	pylibscrypt/tests.py
Disallow NUL in scrypt_mcf passwords

Since libscrypt can't handle them, lets disallow to stay compatible.
Version 1.1.3
Merge branch 'b1.1.x'

Conflicts:
	pylibscrypt/__init__.py
	pylibscrypt/mcf.py
	pylibscrypt/pylibscrypt.py
	pylibscrypt/tests.py