ffmpeg: fix invalid NULL pointer access
ffmpeg: fix invalid NULL pointer access

Catched on Ubuntu 12.04 with FFMPEG:
  codec:                     YES (ver 53.35.0)
  format:                    YES (ver 53.21.1)
  util:                      YES (ver 51.22.3)
  swscale:                   YES (ver 2.1.0)
  resample:                  NO
  gentoo-style:              YES

opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv