net: dsa: hellcreek: Check return value

Return NULL if we got unexpected value from skb_trim_rcsum()
in hellcreek_rcv()

Fixes: 01ef09caad ("net: dsa: Add tag handling for Hirschmann Hellcreek switches")
Signed-off-by: Artem Chernyshev <artem.chernyshev@red-soft.ru>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Link: https://lore.kernel.org/r/20221201140032.26746-2-artem.chernyshev@red-soft.ru
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/dsa/tag_hellcreek.c

@@ -49,7 +49,8 @@ static struct sk_buff *hellcreek_rcv(struct sk_buff *skb,
 		return NULL;
 	}
 
-	pskb_trim_rcsum(skb, skb->len - HELLCREEK_TAG_LEN);
+	if (pskb_trim_rcsum(skb, skb->len - HELLCREEK_TAG_LEN))
+		return NULL;
 
 	dsa_default_offload_fwd_mark(skb);