Fix the ipfw service status output when ipfw.ko isn't loaded

Reported by:	lme
Reviewed by:	lme
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D27657

libexec/rc/rc.d/ipfw

@@ -129,7 +129,9 @@ ipfw_status()
 	status=$(sysctl -i -n net.inet.ip.fw.enable)
 	: ${status:=0}
 	if afexists inet6; then
-		status=$((${status} + $(sysctl -i -n net.inet6.ip6.fw.enable)))
+		status6=$(sysctl -i -n net.inet6.ip6.fw.enable)
+		: ${status6:=0}
+		status=$((${status} + ${status6}))
 	fi
 	if [ ${status} -eq 0 ]; then
 		echo "ipfw is not enabled"