Fix memory leak in _X509_REVOKED_dup

The call to X509_REVOKED_new() will create two empty ASN1 structures
that were never freed, but simply replaced by a copy.
When doing multiple calls to CRL.get_revoked() on a big CRL, this
results in a huge memory leak.
This change adds two calls to free those empty ASN1 structures before
replacing them with the copy.

It requires https://github.com/pyca/cryptography/pull/830 to work.