crypto: aead - Do not allow authsize=0 if auth. alg has digestsize>0

Return -EINVAL on an attempt to set the authsize to 0 with an auth.
algorithm with a non-zero digestsize (i.e. anything but digest_null)
as authenticating the data and then throwing away the result does not
make any sense at all.

The digestsize zero exception is for use with digest_null for testing
purposes only.
Signed-off-by: Pascal van Leeuwen <pvanleeuwen@verimatrix.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/aead.c

@@ -70,7 +70,8 @@ int crypto_aead_setauthsize(struct crypto_aead *tfm, unsigned int authsize)
 {
 	int err;
 
-	if (authsize > crypto_aead_maxauthsize(tfm))
+	if ((!authsize && crypto_aead_maxauthsize(tfm)) ||
+	    authsize > crypto_aead_maxauthsize(tfm))
 		return -EINVAL;
 
 	if (crypto_aead_alg(tfm)->setauthsize) {