examples/l2fwd-crypto: fix AEAD tests when AAD is zero
examples/l2fwd-crypto: fix AEAD tests when AAD is zero

For AEAD algorithms, additional authenticated data (AAD)
can be passed, but it is optional, so its size can be zero.
However, it is required to set this length to zero in the crypto
operation to avoid undefined behaviour.

Fixes: 617a7949c98a ("examples/l2fwd-crypto: parse AAD parameter")
Cc: stable@dpdk.org

Signed-off-by: Pablo de Lara <28355d9cfe41bb8148aea0fb24f47aae8f4217a1@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk