net: if: Make sure iface->if_dev is not null when accessing L2

commit 971ae599 ("net: pkt: Make sure iface is not null when
accessing L2") fixed net_if_l2 where iface was NULL, however if
iface->if_dev is NULL, the check breaks and returns an offset of
NULL (0x82 or so).  This is incorrect.

Let's add a check for iface->if_dev as well.

Signed-off-by: Michael Scott <mike@foundries.io>