odhcp6c: Fix address assignment with infinite validity

git-svn-id: svn://svn.openwrt.org/openwrt@35258 3c298f89-4303-0410-b956-a3cf2f4a3e73
netifd: Fix segfaults in IPv6 prefix handling

git-svn-id: svn://svn.openwrt.org/openwrt@35259 3c298f89-4303-0410-b956-a3cf2f4a3e73