In the start_frame() debug log, print the reference pos value rather than the evaluated value converted to int.

That's required because -1 is evaluated as NAN, which converted back
to int looks like a random number, this is especially annoying when
debugging sources with undefined pos (as the video4linux2 device).
Signed-off-by: Janne Grunau <janne-ffmpeg@jannau.net>