ipv6: fix NULL deref in ip6_rcv_core()

idev can be NULL, as the surrounding code suggests.

Fixes: 4daf841a ("net: ipv6: add skb drop reasons to ip6_rcv_core()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Menglong Dong <imagedong@tencent.com>
Cc: Jiang Biao <benbjiang@tencent.com>
Cc: Hao Peng <flyingpeng@tencent.com>
Link: https://lore.kernel.org/r/20220413205653.1178458-1-eric.dumazet@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>