Fixed, _load_oid wasn't cleared if loadBefore returned None.

src/ZEO/ClientStorage.py

@@ -845,9 +845,8 @@ class ClientStorage(object):
 
             result = self._server.loadBefore(oid, tid)
 
-            if result:
             with self._lock:    # for atomic processing of invalidations
-                    if self._load_status:
+                if result and self._load_status:
                     data, tid, end = result
                     self._cache.store(oid, tid, end, data)
                 self._load_oid = None