This code checked AVFrame.buf[0] instead of the decode return code to
see whether a frame was decoded. This is sort of suspicious; while I
think that the lavc API actually guarantees it, it's not intuitive
anyway. In addition, the code was unnecessarily roundabout.
Replace it with a proper error code check. Remove the other error return
(that was, or should have been, redundant before). The no-frame path is
now cleanly separated. Add an assert on the frame-returned path; if this
fails, lavc violated its own API.