Fixing uncaught exception problem on shutdown

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

doc/HISTORY.rst

 Historical ZODB Changelog
 #########################
 
+- Cleanup of nested mvcc connections could have failed.
+
 3.10.5 (2011-11-19)
 ===================
 

src/ZODB/Connection.py

@@ -1085,6 +1085,7 @@ class Connection(ExportImport, object):
     def _release_resources(self):
         for c in six.itervalues(self.connections):
             if c._mvcc_storage:
+                if c._storage is not None:
                     c._storage.release()
             c._storage = c._normal_storage = None
             c._cache = PickleCache(self, 0, 0)