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