napi_alloc_skb() can return NULL.
We should not crash should this happen.

Fixes: 93f93a44 ("net: move skb_mark_napi_id() into core networking stack")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>