ax25: zero length frame filtering in AX25

In previous commit 244f46ae
was introduced a zero length frame filter for ROSE protocole.
This patch has the same purpose at AX25 frame level for the same
reason. Empty frames have no meaning in AX25 protocole.
Signed-off-by: NBernard Pidoux <f6bvp@amsat.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ax25/af_ax25.c

@@ -1435,6 +1435,11 @@ static int ax25_sendmsg(struct kiocb *iocb, struct socket *sock,
 	size_t size;
 	int lv, err, addr_len = msg->msg_namelen;
 
+	/* AX.25 empty data frame has no meaning : don't send */
+	if (len == 0) {
+		return (0);
+	}
+
 	if (msg->msg_flags & ~(MSG_DONTWAIT|MSG_EOR|MSG_CMSG_COMPAT))
 		return -EINVAL;
 
@@ -1634,6 +1639,13 @@ static int ax25_recvmsg(struct kiocb *iocb, struct socket *sock,
 	skb_reset_transport_header(skb);
 	copied = skb->len;
 
+	/* AX.25 empty data frame has no meaning : ignore it */
+	if (copied == 0) {
+		err = copied;
+		skb_free_datagram(sk, skb);
+		goto out;
+	}
+
 	if (copied > size) {
 		copied = size;
 		msg->msg_flags |= MSG_TRUNC;