i40e: don't free nonexistent rings

Not all VSIs have rings! Check to see if rings were actually allocated before
freeing them.

This prevents a panic when tx_rings[0] is not allocated.
Signed-off-by: NMitch Williams <mitch.a.williams@intel.com>
Signed-off-by: NJesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: NSibai Li <sibai.li@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/i40e/i40e_main.c

@@ -5160,6 +5160,7 @@ static s32 i40e_vsi_clear_rings(struct i40e_vsi *vsi)
 {
 	int i;
 
+	if (vsi->tx_rings[0])
 		for (i = 0; i < vsi->alloc_queue_pairs; i++) {
 			kfree_rcu(vsi->tx_rings[i], rcu);
 			vsi->tx_rings[i] = NULL;