networkAgent: Fix external-ui dialogs

Two small fixes which made _showNewStyleDialog() err out:
 - g_key_file_load_from_data() expects a string as first
   argument, but g_buffered_input_stream_peek_buffer()
   returns an array of "data"
 - g_key_file_load_from_data() is documented to allow -1 as
   length parameter for \0-terminated strings, but the actual
   type of the parameter is unsigned (d'uh)

https://bugzilla.gnome.org/show_bug.cgi?id=671556

js/ui/networkAgent.js

@@ -531,7 +531,8 @@ const VPNRequestHandler = new Lang.Class({
         let contentOverride;
 
         try {
-            keyfile.load_from_data(this._dataStdout.peek_buffer(), -1,
+            let data = this._dataStdout.peek_buffer();
+            keyfile.load_from_data(data.toString(), data.length,
                                    GLib.KeyFileFlags.NONE);
 
             if (keyfile.get_integer(VPN_UI_GROUP, 'Version') != 2)