RDMA/cxgb4: Fix null pointer dereference on alloc_skb failure

Currently if alloc_skb fails to allocate the skb a null skb is passed to
t4_set_arp_err_handler and this ends up dereferencing the null skb.  Avoid
the NULL pointer dereference by checking for a NULL skb and returning
early.

Addresses-Coverity: ("Dereference null return")
Fixes: b38a0ad8 ("RDMA/cxgb4: Set arp error handler for PASS_ACCEPT_RPL messages")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Potnuri Bharat Teja <bharat@chelsio.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/hw/cxgb4/cm.c

@@ -460,6 +460,8 @@ static struct sk_buff *get_skb(struct sk_buff *skb, int len, gfp_t gfp)
 		skb_reset_transport_header(skb);
 	} else {
 		skb = alloc_skb(len, gfp);
+		if (!skb)
+			return NULL;
 	}
 	t4_set_arp_err_handler(skb, NULL, NULL);
 	return skb;