fix forwarding packets when ndp_get_ll_address() returns NULL

When sending a packet a workaround is applied if ndp_get_ll_address() returns NULL as is's not implemented properly yet. [ip.c](https://github.com/RIOT-OS/RIOT/blob/master/sys/net/network_layer/sixlowpan/ip.c#L83)

The same should happen when a packet is forwarded, currently it's dropped which breaks routing.