Initialize the on_fragment callback pointer

http_parser_init() did not initialize the http_parser.on_fragment.

http_parser.rl

@@ -427,6 +427,7 @@ http_parser_init (http_parser *parser, enum http_parser_type type)
   parser->on_path = NULL;
   parser->on_query_string = NULL;
   parser->on_uri = NULL;
+  parser->on_fragment = NULL;
   parser->on_header_field = NULL;
   parser->on_header_value = NULL;
   parser->on_headers_complete = NULL;