irda: small read beyond end of array in debug code

charset comes from skb->data.  It's a number in the 0-255 range.
If we have debugging turned on then this could cause a read beyond
the end of the array.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>