Bluetooth: Fix crash when monitor timeout expires

The code was crashing due to a invalid access to hci_conn after the
channel disconnect.
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Reviewed-by: João Paulo Rechi Vita <jprvita@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/l2cap.c

@@ -1568,6 +1568,9 @@ static struct sk_buff *l2cap_create_iframe_pdu(struct sock *sk, struct msghdr *m
 
 	BT_DBG("sk %p len %d", sk, (int)len);
 
+	if (!conn)
+		return ERR_PTR(-ENOTCONN);
+
 	if (sdulen)
 		hlen += 2;