The NULL pointer check `!ifa' should come before its first use.

[ Bug origin : commit fd23c3b3
  (ipv4: Add hash table of interface addresses) in linux-2.6.39 ]
Signed-off-by: NXi Wang <xi.wang@gmail.com>
Acked-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>