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: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>