fix POSKeyError during commit after savepoint rollback

see https://github.com/zopefoundation/ZODB/issues/16

src/ZODB/Connection.py

@@ -997,6 +997,7 @@ class Connection(ExportImport, object):
     def readCurrent(self, ob):
         assert ob._p_jar is self
         assert ob._p_oid is not None and ob._p_serial is not None
+        if ob._p_serial != z64:
             self._readCurrent[ob._p_oid] = ob._p_serial
 
     # persistent.interfaces.IPersistentDatamanager