ice: fix NULL pointer dereference in ice_update_vsi_tx_ring_stats()

ice: fix NULL pointer dereference in ice_update_vsi_tx_ring_stats()

It is possible to do NULL pointer dereference in routine that updates
Tx ring stats. Currently only stats and bytes are updated when ring
pointer is valid, but later on ring is accessed to propagate gathered Tx
stats onto VSI stats.

Change the existing logic to move to next ring when ring is NULL.

Fixes: e72bba21355d ("ice: split ice_ring onto Tx/Rx separate structs")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Acked-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Tested-by: Gurucharan G <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>