sfc_ef100: potential dereference of null pointer

The return value of kmalloc() needs to be checked.
To avoid use in efx_nic_update_stats() in case of the failure of alloc.

Fixes: b593b6f1 ("sfc_ef100: statistics gathering")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sfc/ef100_nic.c

@@ -609,6 +609,9 @@ static size_t ef100_update_stats(struct efx_nic *efx,
 	ef100_common_stat_mask(mask);
 	ef100_ethtool_stat_mask(mask);
 
+	if (!mc_stats)
+		return 0;
+
 	efx_nic_copy_stats(efx, mc_stats);
 	efx_nic_update_stats(ef100_stat_desc, EF100_STAT_COUNT, mask,
 			     stats, mc_stats, false);