Previously this was not tested and excepted KERNEL_PID_UNDEF == 0 as legal value for received `gnrc_netif_hdr`s (which except for the loopback interface is wrong)
Previously this was not tested and excepted KERNEL_PID_UNDEF == 0 as legal value for received `gnrc_netif_hdr`s (which except for the loopback interface is wrong)