net: remove unnecessary NULL checking in napi_consume_skb()

When budget is non-zero, skb_unref() has already handled the
NULL checking.

When budget is zero, the dev_consume_skb_any() has handled NULL
checking in __dev_kfree_skb_irq(), or dev_kfree_skb() which also
ultimately call skb_unref().

So remove the unnecessary checking in napi_consume_skb().

Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>