network: Fix reference leak of NetProxy

Every time we got the list of devices from the devices GtkListStore, we
were leaking the only element that wasn't a NetDevice, the proxy page.
Make sure to unref' it by hand if we're not going to add it to the list
(the elements of which will be unref'ed properly).

https://bugzilla.gnome.org/show_bug.cgi?id=746411