atm: clip: prevent NULL deref in clip_push()

[ Upstream commit b993ea46 ]

Blamed commit missed that vcc_destroy_socket() calls
clip_push() with a NULL skb.

If clip_devs is NULL, clip_push() then crashes when reading
skb->truesize.

Fixes: 93a2014a ("atm: fix a UAF in lec_arp_clear_vccs()")
Reported-by:  <syzbot+1316233c4c6803382a8b@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/netdev/68556f59.a00a0220.137b3.004e.GAE@google.com/T/#u


Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Gengming Liu <l.dmxcsnsbh@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>