Fix undefined behavior wrt memcpy
* cipher/cipher-gcm.c: Do not copy zero bytes from an empty buffer. Let the function continue to add padding as needed though. * cipher/mac-poly1305.c: If the caller requested to finish the hash function without a copy of the result, return immediately.
Caught by UndefinedBehaviorSanitizer.
- Signed-off-by: Peter Wu <peter@lekensteyn.nl>