iavf: validate pointers

In some cases, the ethtool get_rxfh handler may be called with a null
key or indir parameter. So check these pointers, or you will have a very
bad day.

Fixes: 43a3d9ba

 ("i40evf: Allow PF driver to configure RSS")
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Tony Brelinski <tony.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>