Exception when converting small videos even if the conversion worked

Hi,

When converting very small videos (for instance 2-3 seconds), the exception "Unknown ffmpeg error" is raised, because the yielded variable did not have the time to be set to True (I think). But the video has correctly been converted!

Line 493 of ffmpeg.py:

``` python
if not yielded:
    raise FFMpegConvertError('Unknown ffmpeg error', cmd,
         total_output, line, pid=p.pid)
```
