Ensure av_write_trailer is only called once (fixes: #613)

We must only ever call av_write_trailer once, otherwise we get a segmentation fault. Therefore no matter whether it succeeds or not we must absolutely set self._done.

Also we need to be more careful when closing the streams' CodecContext:

• It's too late to iterate over the streams in __dealloc__ as Python object destruction may have already started.

• We must not error if there is not CodecContext or if the CodecContext has already been closed by the user.