Merge commit '44129e38'

* commit '44129e38':
  avconv: Do not pass NULL to avio_tell

This commit is a noop. The FFmpeg code handle cases where total_size is
an AVERROR (which avio_tell(NULL) will return) later in the function.

Note: the original commit refers to the null *muxer*. `ffmpeg -lavfi
testsrc -f null -` can be used as a test case.
Merged-by: Clément Bœsch <u@pkh.me>