client: fix "snapshot-less" use of Storage

It was broken since commit 4a4d2e64
("client: optimize by not calling lastTransaction at the end of transactions").

This fixes several unit tests like
neo.tests.zodb.testBasic.BasicTests.check_tid_ordering_w_commit

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2819 71dcc9de-d417-0410-9af5-da40c76e7ee4

neo/client/Storage.py

@@ -128,7 +128,8 @@ class Storage(BaseStorage.BaseStorage,
     def tpc_finish(self, transaction, f=None):
         tid = self.app.tpc_finish(transaction=transaction,
             tryToResolveConflict=self.tryToResolveConflict, f=f)
-        self._snapshot_tid = add64(tid, 1)
+        if self._snapshot_tid:
+            self._snapshot_tid = add64(tid, 1)
         return tid
 
     @check_read_only