i40e: prevent null pointer exception in dump descriptor

Check that rings were allocated before trying to reference them.

Change-Id: I33151e55ab7a7a305fecdb88ccb2709ac246b7c7
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>