fixed the bug that makes wrong closing in JsonParserAgent::read().

The function uses json_reader_read_element().
So correspoing closing function is json_reader_end_element().

The reason why code works normally even with the wrong function
is unknown. But the previous code clearly violates the usage of
JSON-GLib.

https://developer.gnome.org/json-glib/0.12/JsonReader.html#json-reader-read-element