ieee802154: fix logic error in ieee802154_llsec_parse_dev_addr

Fix a logic error to avoid potential null pointer dereference.

Signed-off-by: Baozeng Ding <sploving1@gmail.com>
Reviewed-by: Stefan <Schmidt&lt;stefan@osg.samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>