GCM tag() can return an unwritten tag; RSA decrypt2 accepts a wrong-length plaintext

[MarkAtwood]

• GCM: after a one-shot gnutls_aead_cipher_decrypt, a decryptv2 on the same handle with empty ciphertext, attacker-chosen AAD and a zero tag is accepted (nettle rejects it). encryptv can emit stale buffer bytes as the tag. TLS is not affected.
• RSA: gnutls_privkey_decrypt_data2 returns success for a plaintext shorter than requested (nettle returns an error).

Fixes and tests: #88 (closed in favour of this issue).