reapply the _fields check, it's needed for CPython 2.5

lib/mako/_ast_util.py

diff --git a/lib/mako/_ast_util.py b/lib/mako/_ast_util.py
index 706425c..6ff4ab9 100644
--- a/lib/mako/_ast_util.py
+++ b/lib/mako/_ast_util.py
@@ -171,6 +171,9 @@
 
 def iter_fields(node):
     """Iterate over all fields of a node, only yielding existing fields."""
+    # CPython 2.5 compat
+    if not hasattr(node, '_fields') or not node._fields:
+        return
     for field in node._fields:
         try:
             yield field, getattr(node, field)