`maried` sometimes crashes on short songs

[dsprenkels]

I get this error during the playback of a sequence of very short songs:

Traceback (most recent call last):
  File "/home/maried/py/maried/gstreamer.py", line 302, in _on_about_to_finish
    self.on_about_to_finish()
  File "/home/maried/py/sarah/event.py", line 8, in __call__
    handler(*args, **kwargs)
  File "/home/maried/py/maried/core.py", line 376, in _player_on_about_to_finish
    self._queue_next()
  File "/home/maried/py/maried/core.py", line 324, in _queue_next
    self.player.queue(media)
  File "/home/maried/py/maried/gstreamer.py", line 218, in queue
    "Another media has been queued already"
RuntimeError: Another media has been queued already
11825 INFO    history:<ClassicRequest sholleman - <ClassicMedia MrPiep - NS error>>
Traceback (most recent call last):
  File "/home/maried/py/maried/gstreamer.py", line 360, in on_message
    self._on_stream_changed()
  File "/home/maried/py/maried/gstreamer.py", line 298, in _on_stream_changed
    self.on_playing_started(self.playing_media, self.endTime)
  File "/home/maried/py/sarah/event.py", line 8, in __call__
    handler(*args, **kwargs)
  File "/home/maried/py/maried/core.py", line 348, in _player_on_playing_started
    assert media == self.next_playing_media
AssertionError
12509 INFO    history:<ClassicRequest sholleman - <ClassicMedia MrPiep - NS error>>