Inadeqate input validation and a reference leak in crypto.{load, dump}_privatekey

[Ziga Seilnacht]

crpto.load_privatekey and crypto.dump_privatekey have a few bugs in uncommonly used paths:
 - they don't raise an error if a passphrase is specified with a serialization format that doesn't support encryption
 - they truncate long passphrases supplied via callback
 - they either overwrite the error raised by the passphrase (load_privatekey) callback or forget to cleanup OpenSSL's error stack in that case (dump_privatekey)
 - they forget to decref the passphrase returned by the callback
 - they are missing a few NULL checks

I'll submit a branch with the fixes.

[Jean-Paul Calderone (exarkun)]

Thanks. Fixed in r161.