browser-plugin: Prevent a copy when checking for valid extension UUIDs

Instead of using g_strndup to copy the NPString that gets passed to us
by the plugin host, just use the equally-as-good NPString directly. We
still need to copy the string when passing it over DBus, as there's no
easy way to construct a string GVariant from a length-prefixed string.

https://bugzilla.gnome.org/show_bug.cgi?id=679099