mac802154: fakelb: Fix potential NULL pointer dereference.

fakelb_hw_deliver creates a copy of the skb's header which can
potentially return NULL so we now check for this before actually
delivering to the 802.15.4 MAC layer.

Signed-off-by: Martin Townsend <martin.townsend@xsilon.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>