AttributeError in the cache when using persistent cache files

[jamadden]

Given a certain cache pattern that resulted in some rings being overfull, it was possible for the in-memory cache to become corrupted when loading data from disk. The result was an AttributeError like this:

2016-11-28 17:30:57,352 ERROR [ZODB.Connection][139897887425392:12185][MainThread] Couldn't load state for persistent.mapping.PersistentMapping 0x01
Traceback (most recent call last):
  File "//buildout/eggs/ZODB-5.1.1-py2.7.egg/ZODB/Connection.py", line 783, in setstate
    p, serial = self._storage.load(oid)
  File "//buildout/eggs/zc.zlibstorage-1.1.0-py2.7.egg/zc/zlibstorage/__init__.py", line 59, in load
    data, serial = self.base.load(oid, version)
  File "//buildout/eggs/perfmetrics-2.0-py2.7.egg/perfmetrics/__init__.py", line 127, in call_with_metric
    return f(*args, **kw)
  File "//buildout/eggs/RelStorage-2.0.0b8-py2.7-linux-x86_64.egg/relstorage/storage.py", line 566, in load
    state, tid_int = cache.load(cursor, oid_int)
  File "//buildout/eggs/RelStorage-2.0.0b8-py2.7-linux-x86_64.egg/relstorage/cache/storage_cache.py", line 451, in load
    response = client.get_multi(cachekeys)
  File "//buildout/eggs/RelStorage-2.0.0b8-py2.7-linux-x86_64.egg/relstorage/cache/local_client.py", line 139, in get_multi
    res = get(keys)
  File "//buildout/eggs/RelStorage-2.0.0b8-py2.7-linux-x86_64.egg/relstorage/cache/mapping.py", line 195, in get_and_bubble_all
    gens[entry.cffi_entry.r_parent].on_hit(entry)
AttributeError: 'NoneType' object has no attribute 'on_hit'