Fix a python 2.6.5 compatibility

The upcoming python 2.6.5 release has a change that makes delattr(obj,
attr) fail for slots-enabled objects if the attr is not already set.

To prevent against this, we only run the delattr if the attribute is
already set.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>

lib/opcodes.py

@@ -90,7 +90,7 @@ class BaseOpCode(object):
                        type(state))
 
     for name in self._all_slots():
-      if name not in state:
+      if name not in state and hasattr(self, name):
         delattr(self, name)
 
     for name in state: