luci-app-upnp: supress stderr when listing MINIUPNPD chain

Current OpenWrt trunk now relays uhttpd stderr to syslog, so don't spam
it with "iptables: No chain/target/match by that name." when miniupnpd
is not configured but its status is queried by an overview page applet.

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