net/mlx4: fix RSS action with null hash key

net/mlx4: fix RSS action with null hash key

If RSS action contains non zero hash key length and NULL
key buffer pointer the default hash key should be used.
The check for the NULL pointer this was missing in the mlx4
PMD causing crash, for example, in testpmd with command:

flow validate 0 ingress group 0
  pattern eth / ipv4 / end
  actions rss queues 0 end key_len 40 / end

Fixes: ac8d22de2394 ("ethdev: flatten RSS configuration in flow API")
Cc: stable@dpdk.org
Signed-off-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>