Bluetooth: 6lowpan: Make sure skb exists before accessing it

We need to make sure that the saved skb exists when
resuming or suspending a CoC channel. This can happen if
initial credits is 0 when channel is connected.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>