/*
 * Closes this reader to disallow further reads.
 *
 *    reader = Bzip2::Reader.new File.open('file')
 *    reader.close
 *
 *    reader.closed? # => true
 *
 * @return [File] the io with which the reader was created.
 * @raise [IOError] if the stream has already been closed
 */
static VALUE bz_reader_close(VALUE obj) {