firewall: fix invalid memory access when processing /128 IPv6 addresses from ubus, properly emit REDIRECT rules for local port forwards

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42604 3c298f89-4303-0410-b956-a3cf2f4a3e73