iavf: check for null in iavf_fix_features

If the driver has lost contact with the PF then it enters a disabled state
and frees adapter->vf_res. However, ndo_fix_features can still be called on
the interface, so we need to check for this condition first. Since we have
no information on the features at this time simply leave them unmodified
and return.

Fixes: c4445aed

 ("i40evf: Fix VLAN features")
Signed-off-by: Nicholas Nunley <nicholas.d.nunley@intel.com>
Tested-by: Tony Brelinski <tony.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>