crypto/aesni_mb: fix zero burst dequeue

crypto/aesni_mb: fix zero burst dequeue

In the unlikely scenario that an application
calls rte_cryptodev_dequeue_burst with nb_ops = 0,
there was a job leak, as a job would be created
but would not be populated, as no operation is passed.

Fixes: 0f548b50a160 ("crypto/aesni_mb: process crypto op on dequeue")
Cc: stable@dpdk.org
Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Declan Doherty <declan.doherty@intel.com>