Merge commit 'e46a6fb7'

* commit 'e46a6fb7':
  avconv: Check that muxing_queue exists before reading from it

Mostly noop. This was fixed in FFmpeg in 7f7c494a.

The merge makes the cosmetics match but does not include the weird
av_log().

Merged-by: Clément Bœsch <cboesch@gopro.com>