crypto: qat - Prevent dma mapping zero length assoc data

Do not attempt to dma map associated data if it is zero length.

Cc: stable@vger.kernel.org
Signed-off-by: Tadeusz Struk <tadeusz.struk@intel.com>
Tested-by: Nikolay Aleksandrov <nikolay@redhat.com>
Reviewed-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>