crypto: aead - add check for presence of auth tag

The AEAD decryption operation requires the authentication tag to be
present as part of the cipher text buffer. The added check verifies that
the caller provides a cipher text with at least the authentication tag.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>