luci-base: properly handle undefined IPv6 local-address information
luci-base: properly handle undefined IPv6 local-address information

If IPv6 prefix assignment is disabled, the "local-address" structure
might exist, but be empty which causes the adress formatting in the
network model class to bail out.

Verify the completeness of the "local-address" structure before using
it in order to avoid runtime errors.

Fixes #1657.
Signed-off-by: Jo-Philipp Wich <bd73d35759d75cc215150d1bbc94f1b1078bee01@mein.io>

tobiaswaldvogel/luci,nmav/luci,981213/luci-1,rogerpueyo/luci,remakeelectric/luci,Noltari/luci,artynet/luci,wongsyrone/luci-1,Noltari/luci,Noltari/luci,nmav/luci,openwrt-es/openwrt-luci,kuoruan/luci,981213/luci-1,hnyman/luci,hnyman/luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,openwrt/luci,rogerpueyo/luci,rogerpueyo/luci,chris5560/openwrt-luci,remakeelectric/luci,981213/luci-1,981213/luci-1,981213/luci-1,tobiaswaldvogel/luci,chris5560/openwrt-luci,openwrt-es/openwrt-luci,openwrt/luci,remakeelectric/luci,kuoruan/lede-luci,wongsyrone/luci-1,artynet/luci,kuoruan/lede-luci,wongsyrone/luci-1,kuoruan/luci,rogerpueyo/luci,openwrt/luci,rogerpueyo/luci,lbthomsen/openwrt-luci,artynet/luci,wongsyrone/luci-1,nmav/luci,kuoruan/luci,tobiaswaldvogel/luci,tobiaswaldvogel/luci,chris5560/openwrt-luci,hnyman/luci,rogerpueyo/luci,981213/luci-1,artynet/luci,remakeelectric/luci,hnyman/luci,nmav/luci,artynet/luci,nmav/luci,kuoruan/lede-luci,remakeelectric/luci,openwrt-es/openwrt-luci,artynet/luci,chris5560/openwrt-luci,kuoruan/lede-luci,remakeelectric/luci,chris5560/openwrt-luci,openwrt-es/openwrt-luci,lbthomsen/openwrt-luci,tobiaswaldvogel/luci,Noltari/luci,Noltari/luci,wongsyrone/luci-1,openwrt/luci,lbthomsen/openwrt-luci,wongsyrone/luci-1,nmav/luci,kuoruan/lede-luci,kuoruan/lede-luci,hnyman/luci,kuoruan/lede-luci,wongsyrone/luci-1,kuoruan/luci,chris5560/openwrt-luci,nmav/luci,openwrt-es/openwrt-luci,remakeelectric/luci,Noltari/luci,kuoruan/lede-luci,kuoruan/luci,hnyman/luci,nmav/luci,tobiaswaldvogel/luci,lbthomsen/openwrt-luci,kuoruan/luci,openwrt/luci,openwrt/luci,artynet/luci,openwrt/luci,nmav/luci,rogerpueyo/luci,tobiaswaldvogel/luci,kuoruan/luci,tobiaswaldvogel/luci,Noltari/luci,981213/luci-1,artynet/luci,artynet/luci,chris5560/openwrt-luci,hnyman/luci,openwrt/luci,Noltari/luci,openwrt-es/openwrt-luci,openwrt-es/openwrt-luci,wongsyrone/luci-1,openwrt-es/openwrt-luci,lbthomsen/openwrt-luci,Noltari/luci,kuoruan/luci,rogerpueyo/luci,hnyman/luci,remakeelectric/luci,chris5560/openwrt-luci