i40evf: ethtool RSS fixes

Add an extra check to make sure that the indirection table pointer is
valid before dereferencing it.

Change-ID: I698adbf3daff03081d01f489dc95a9f1ad8b12f1
Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>