net/ip: apply workaround to fix IP forwarding

When sending a packet a workaround is applied when ndp_get_ll_address() returns NULL as is's not implemented properly yet.
The same should happen when a packet is forwarded as it's dropped otherwise.
net/ip: fix wrong double negative in the comments
Merge pull request #1046 from benpicco/fix_routing

fix forwarding packets when ndp_get_ll_address() returns NULL