mISDN: array underflow in open_bchannel()

There are two channels here.  User space starts counting channels at one
but in the kernel we start at zero.  If the user passes in a zero
channel that's invalid and could lead to memory corruption.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/isdn/hardware/mISDN/netjet.c b/drivers/isdn/hardware/mISDN/netjet.c
index dd6de9f..c726e09 100644
--- a/drivers/isdn/hardware/mISDN/netjet.c
+++ b/drivers/isdn/hardware/mISDN/netjet.c
@@ -860,7 +860,7 @@
 {
 	struct bchannel *bch;
 
-	if (rq->adr.channel > 2)
+	if (rq->adr.channel == 0 || rq->adr.channel > 2)
 		return -EINVAL;
 	if (rq->protocol == ISDN_P_NONE)
 		return -EINVAL;