net: core: Free packet properly if TTL/hop limit is 0

We drop the packet if TTL or hop limit is 0, but we should
also unref the packet in this case because we return 0 to
the caller which is not then able to free the packet because
it thinks that the packet was sent properly.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>