libupnp: security update to 1.6.18

Signed-off-by: Russell Senior <russell@personaltelco.net>
Signed-off-by: Tim Yardley <yardley@gmail.com>



git-svn-id: svn://svn.openwrt.org/openwrt/packages@35522 3c298f89-4303-0410-b956-a3cf2f4a3e73