net: pkt: Make sure iface is not null when accessing L2

It is possible that the network interface is not set when we
check the interface in net_pkt.c:pkt_buffer_length(). For example
in icmpv6 unit test the interface is left as NULL as the test does
not care about what network interface is used. For real hw like
mimxrt1050_evk, which supports Ethernet, we need to add additional
checks for the interface being non-null.

Fixes #20088

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>