GCM: Add clearing and checking of marks.tag
* cipher/cipher-gcm.c (_gcry_cipher_gcm_encrypt) (_gcry_cipher_gcm_decrypt, _gcry_cipher_gcm_authenticate): Make sure that tag has not been finalized yet. (_gcry_cipher_gcm_setiv): Clear 'marks.tag'.
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>