Chacha20poly1305 enc/dec function not symmetrical

[sacca97]

Not sure you intended this, but the encryption and decryption function are not symmetrical. (i.e. the ct length is not encoded in the padding)

Basic code sample follows.

` uint8_t msg[7] = "Simola";
size_t pad = (sizeof(msg) + 15) & 0xFFFFFFF0;
uint8_t enc[16 + pad];
aead_encrypt(enc, msg, 7, NULL, 0, 0, key_bytes);
uint8_t dec[16];
int rc = aead_decrypt(dec, enc, sizeof(enc), NULL, 0, 0, key_bytes);

assert(crypto_equal(msg, dec, 7) && rc == 1);`

The assertion fails, but If I call the decryption function with srclen = 16+7 then the decryption is successfull.