kdf:argon2: Fix for the case output > 64.

* cipher/blake2.c (blake2b_vl_hash): Fix the last step.
* cipher/kdf.c (argon2_open): Check the value.

Cherry-picked master commit of:
564739a58426d89db2f0c9334659949e503d2c59

• Reported-by: Guido Vranken <guidovranken@gmail.com>
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>