af_packet: Quiet sparse noise about using plain integer as NULL pointer

Quiets the sparse warning:
warning: Using plain integer as NULL pointer
Signed-off-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -1079,7 +1079,7 @@ static void *packet_current_rx_frame(struct packet_sock *po,
 	default:
 		WARN(1, "TPACKET version not supported\n");
 		BUG();
-		return 0;
+		return NULL;
 	}
 }