ffmpeg.c: fix erroneous "Output file is empty" message.

This message is printed whenever the RAWVIDEO codec is used, whether the
output file contains any frames or not. Test command:
./ffmpeg -i infile.y4m outfile.y4m

Signed-off-by: Mike Scheutzow <mjs973@optonline.net>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>