Fixed an error in the Zip library that didn't allow downloading on PHP 4 servers.
diff --git a/system/libraries/Zip.php b/system/libraries/Zip.php
index f81da85..4ed1f18 100644
--- a/system/libraries/Zip.php
+++ b/system/libraries/Zip.php
@@ -325,11 +325,13 @@
 			$filename .= '.zip';
 		}
 
-		$zip_content =& $this->get_zip();
-
 		$CI =& get_instance();
 		$CI->load->helper('download');
 
+		$get_zip = $this->get_zip();
+
+		$zip_content =& $get_zip;
+
 		force_download($filename, $zip_content);
 	}