This fixes the following assertion failure:

PACKET #0x021d Ping                           > M1 (10.0.114.64:14001)
INFO   timeout for #0x0000021d with <ClientConnection(uuid=M1, address=10.0.114.64:14001, closed=0, handler=BackupHandler) at 2955590>
DEBUG  connection closed for <ClientConnection(uuid=M1, address=10.0.114.64:14001, closed=1, handler=BackupHandler) at 2955590>
ERROR  upstream master is down: connection lost
ERROR  Pre-mortem data:
ERROR  Traceback (most recent call last):
ERROR    File "neo/master/app.py", line 134, in run
ERROR      self._run()
ERROR    File "neo/master/app.py", line 154, in _run
ERROR      self.playPrimaryRole()
ERROR    File "neo/master/app.py", line 344, in playPrimaryRole
ERROR      self.backup_app.provideService()
ERROR    File "neo/master/backup_app.py", line 129, in provideService
ERROR      conn.close()
ERROR    File "neo/lib/connection.py", line 541, in close
ERROR      assert not self.isPending()
ERROR  AssertionError