Fixing uncaught exception problem on shutdown

Nested connections try to clean up themselves multiple
times. This does not work

src/CHANGES.txt

@@ -18,6 +18,8 @@ Bugs Fixed
 - Pinned the ``transaction`` and ``manuel`` dependencies to Python 2.5-
   compatible versions when installing under Python 2.5.
 
+- Cleanup of nested mvcc connections could have failed.
+
 3.10.5 (2011-11-19)
 ===================
 

src/ZODB/Connection.py

@@ -1073,7 +1073,8 @@ class Connection(ExportImport, object):
     def _release_resources(self):
         for c in self.connections.itervalues():
             if c._mvcc_storage:
-                c._storage.release()
+                if c._storage is not None:
+                    c._storage.release()
             c._storage = c._normal_storage = None
             c._cache = PickleCache(self, 0, 0)