3c515: Write outside array bounds

if dev_alloc_skb() fails on the first iteration, a write to
cp->rx_ring[-1] occurs.
Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/3c515.c

@@ -832,7 +832,9 @@ static int corkscrew_open(struct net_device *dev)
 			skb_reserve(skb, 2);	/* Align IP on 16 byte boundaries */
 			vp->rx_ring[i].addr = isa_virt_to_bus(skb->data);
 		}
-		vp->rx_ring[i - 1].next = isa_virt_to_bus(&vp->rx_ring[0]);	/* Wrap the ring. */
+		if (i != 0)
+			vp->rx_ring[i - 1].next =
+				isa_virt_to_bus(&vp->rx_ring[0]);	/* Wrap the ring. */
 		outl(isa_virt_to_bus(&vp->rx_ring[0]), ioaddr + UpListPtr);
 	}
 	if (vp->full_bus_master_tx) {	/* Boomerang bus master Tx. */