Bug in pulsar datastore lpush command

[lsbardel]

While developing pulsar-queue I discovered this:

20:21:02 [p=3848, t=140735267885824, ERROR, pulsar.arbiter] Server error on 'lpush' command
Traceback (most recent call last):
  File "/usr/local/lib/python3.4/site-packages/pulsar/apps/ds/client.py", line 89, in _execute_command
    handle(self, request, len(request) - 1)
  File "/usr/local/lib/python3.4/site-packages/pulsar/apps/ds/server.py", line 1168, in lpush
    self._signal(self.NOTIFY_LIST, db, request[0], key, N - 1)
  File "/usr/local/lib/python3.4/site-packages/pulsar/apps/ds/server.py", line 2437, in _signal
    self._event_handlers[type](db, key, COMMANDS_INFO[command])
  File "/usr/local/lib/python3.4/site-packages/pulsar/apps/ds/server.py", line 2479, in _list_event
    client.blocked.unblock(client, key, value)
  File "/usr/local/lib/python3.4/site-packages/pulsar/apps/ds/client.py", line 250, in unblock
    value, self.dest)
  File "/usr/local/lib/python3.4/site-packages/pulsar/apps/ds/server.py", line 2191, in _block_callback
    elem = value.pop()
IndexError: pop from an empty deque