firewall: fix potential NULL pointer access

Properly skip struct ifaddr entries with NULL ifa_addr, thanks Kostas Papadopoulos for reporting.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42138 3c298f89-4303-0410-b956-a3cf2f4a3e73