Bugfix: Remove assert from Certificate.from_p12

[priitlatt]

PKCS#12 containers can be valid even if they do not contain any certificate entries in them. Furthermore, cryptography's built-in pkcs12.load_key_and_certificates, which is used by Certificate.from_p12, has a return type Optional[x509.Certificate] for the returned certificate.

The factory method however expected that the given PKCS#12 container always contains the certificate, which is a false assumption, and did a mere assert x509_certificate is not None. This can lead to unexpected results:

• in production mode assertions are ignored altogether and a snowball effect could arise with issues deeper down the line,
• unexpected AssertionError is thrown in case assertions are enabled.

To overcome this, check that the certificate acquired from the container is usable, and raise a descriptive ValueError otherwise.