ice: Check root pointer for validity

ice_sched_get_tc_node uses pi->root without checking for NULL. Add a
check to prevent NULL pointer dereference.
Signed-off-by: NAnirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: NAndrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/ice/ice_sched.c

@@ -284,7 +284,7 @@ struct ice_sched_node *ice_sched_get_tc_node(struct ice_port_info *pi, u8 tc)
 {
 	u8 i;
 
-	if (!pi)
+	if (!pi || !pi->root)
 		return NULL;
 	for (i = 0; i < pi->root->num_children; i++)
 		if (pi->root->children[i]->tc_num == tc)