kdf: handle errors from thread dispatch/wait functions

* cipher/kdf.c (argon2_compute): Handle failed job dispatch/wait.
* tests/t-kdf.c (pthread_jobs_launch_job)
(wait_all_jobs_completion): Handle errors returned from pthread functions.

This allows thread helpers to return error code, which causes
KDF processing to stop.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>