This fixes the following crash (for example when a client disconnects during
tpc_finish):

Traceback (most recent call last):
  ...
  File "neo/master/handlers/storage.py", line 68, in answerInformationLocked
    self.app.tm.lock(ttid, conn.getUUID())
  File "neo/master/transactions.py", line 338, in lock
    if self._ttid_dict[ttid].lock(uuid) and self._queue[0][1] == ttid:
IndexError: list index out of range