atm: clip: prevent NULL deref in clip_push()

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: 93a2014 ("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>

Author: edumazet

net/atm/clip.c

@@ -193,12 +193,6 @@ static void clip_push(struct atm_vcc *vcc, struct sk_buff *skb)
 
 	pr_debug("\n");
 
-	if (!clip_devs) {
-		atm_return(vcc, skb->truesize);
-		kfree_skb(skb);
-		return;
-	}
-
 	if (!skb) {
 		pr_debug("removing VCC %p\n", clip_vcc);
 		if (clip_vcc->entry)
@@ -208,6 +202,11 @@ static void clip_push(struct atm_vcc *vcc, struct sk_buff *skb)
 		return;
 	}
 	atm_return(vcc, skb->truesize);
+	if (!clip_devs) {
+		kfree_skb(skb);
+		return;
+	}
+
 	skb->dev = clip_vcc->entry ? clip_vcc->entry->neigh->dev : clip_devs;
 	/* clip_vcc->entry == NULL if we don't have an IP address yet */
 	if (!skb->dev) {