Fix crash in `tf.contrib.ffmpeg.decode_video`

This fix fixes the crash in `tf.contrib.ffmpeg.decode_video`.
The reason for the crash was that, `decode_video` dumps
the information about streaming etc. (as opposed to dump
to stderr) info a file and read from it. As the loglevel
was `error` the file was empty.

This fix addresses the issue.

The fix could be verifed by manually running
```
bazel test -s --config=opt --cache_test_results=no //tensorflow/contrib/ffmpeg:decode_video_op_test
```

With this fix, the above test works.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>