This is similar to 20a5956b

The difference this change makes affects only rather few files
but looks like:
Stream #0:1: Video: mjpeg, 90k tbr, 90k tbn, 90k tbc
vs.
Stream #0:1: Video: mjpeg, none(bt470bg/unknown/unknown), 90k tbr, 90k tbn, 90k tbc
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>