Fix crash in theano-cache purge. fix gh-2428.

theano/gof/cmodule.py

@@ -666,6 +666,7 @@ class ModuleCache(object):
         # Go through directories in alphabetical order to ensure consistent
         # behavior.
         subdirs = sorted(os.listdir(self.dirname))
+        files, root = None, None  # To make sure the del wor
         for root in subdirs:
             root = os.path.join(self.dirname, root)
             key_pkl = os.path.join(root, 'key.pkl')