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: e72bba21 ("ice: split ice_ring onto Tx/Rx separate structs")
Reported-by: Nkernel test robot <lkp@intel.com>
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NMaciej Fijalkowski <maciej.fijalkowski@intel.com>
Acked-by: NAlexander Lobakin <alexandr.lobakin@intel.com>
Tested-by: Gurucharan G <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: NTony Nguyen <anthony.l.nguyen@intel.com>