Use 'bytes' (the return value from af_alg_final) instead of 'res'
(which is 0 after the last successful af_alg_update call) when
copying the digest.
This bug caused the memcpy to copy 0 bytes, resulting in an
uninitialized digest buffer. It also triggered a GCC
-Werror=stringop-overflow warning because 'res' could theoretically
be negative, leading to a huge unsigned size.
6c6e7251b2